@article{pope_mori_abdelmaguid_gelfand_reynolds_safi-harb_hailey_an_bangale_batista_et al._2024, title={A Multiwavelength Investigation of PSR J2229+6114 and its Pulsar Wind Nebula in the Radio, X-Ray, and Gamma-Ray Bands}, volume={960}, ISSN={["1538-4357"]}, url={https://doi.org/10.3847/1538-4357/ad0120}, DOI={10.3847/1538-4357/ad0120}, abstractNote={G106.3+2.7, commonly considered to be a composite supernova remnant (SNR), is characterized by a boomerang-shaped pulsar wind nebula (PWN) and two distinct (“head” and “tail”) regions in the radio band. A discovery of very-high-energy gamma-ray emission (E γ > 100 GeV) followed by the recent detection of ultrahigh-energy gamma-ray emission (E γ > 100 TeV) from the tail region suggests that G106.3+2.7 is a PeVatron candidate. We present a comprehensive multiwavelength study of the Boomerang PWN (100″ around PSR J2229+6114) using archival radio and Chandra data obtained two decades ago, a new NuSTAR X-ray observation from 2020, and upper limits on gamma-ray fluxes obtained by Fermi-LAT and VERITAS observatories. The NuSTAR observation allowed us to detect a 51.67 ms spin period from the pulsar PSR J2229+6114 and the PWN emission characterized by a power-law model with Γ = 1.52 ± 0.06 up to 20 keV. Contrary to the previous radio study by Kothes et al., we prefer a much lower PWN B-field (B ∼ 3 μG) and larger distance (d ∼ 8 kpc) based on (1) the nonvarying X-ray flux over the last two decades, (2) the energy-dependent X-ray size of the PWN resulting from synchrotron burn-off, and (3) the multiwavelength spectral energy distribution (SED) data. Our SED model suggests that the PWN is currently re-expanding after being compressed by the SNR reverse shock ∼1000 yr ago. In this case, the head region should be formed by GeV–TeV electrons injected earlier by the pulsar propagating into the low-density environment.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Pope, I. and Mori, K. and Abdelmaguid, M. and Gelfand, J. D. and Reynolds, S. P. and Safi-Harb, S. and Hailey, C. J. and An, H. and Bangale, P. and Batista, P. and et al.}, year={2024}, month={Jan} }
 @article{reynolds_borkowski_2024, title={An X-Ray Synchrotron Shell and a Pulsar: The Peculiar Supernova Remnant G32.4+0.1}, volume={962}, ISSN={["1538-4357"]}, url={https://doi.org/10.3847/1538-4357/ad1a16}, DOI={10.3847/1538-4357/ad1a16}, abstractNote={
 We present a deep Chandra observation of the shell supernova remnant (SNR) G32.4+0.1, whose featureless X-ray spectrum has led to its classification as an X-ray synchrotron-dominated SNR. We find a partial shell morphology whose outline is quite circular, with a radius of about 11 pc at an assumed distance of 11 kpc. Thermal and power-law spectral models for three relatively bright regions provided equally good fits, but the absence of spectral lines required ionization timescales from thermal fits that are inconsistent with the mean densities derived from emission measures. We thus confirm the nonthermal, i.e., synchrotron, origin of X-rays from G32.4+0.1. Shock velocities needed to accelerate electrons to the required TeV energies are ≳1000 km s−1, giving a remnant age ≲ 5000–9000 yr. There is no obvious X-ray counterpart to the radio pulsar PSR J1850−0026, but its position adjoins a region of X-ray emission whose spectrum is somewhat harder than that of other regions of the shell, and which may be a pulsar-wind nebula (PWN), though its spectrum is steeper than almost all known X-ray PWNe. The distance of the pulsar from the center of symmetry of the shell disfavors a birth in a supernova event at that location only a few thousand years before; either the pulsar (and putative PWN) are not associated with the shell SNR, requiring a coincidence of both position and (roughly) absorbing column density, or the SNR is much older, making the origin of nonthermal emission problematic.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Reynolds, Stephen P. and Borkowski, Kazimierz J.}, year={2024}, month={Feb} }
 @article{kim_park_woo_silverman_an_bamba_mori_reynolds_safi-harb_2024, title={X-Ray Characterization of the Pulsar PSR J1849-0001 and Its Wind Nebula G32.64+0.53 Associated with TeV Sources Detected by HESS, HAWC, Tibet AS<i>γ</i>, and LHAASO}, volume={960}, ISSN={["1538-4357"]}, url={https://doi.org/10.3847/1538-4357/ad0ecd}, DOI={10.3847/1538-4357/ad0ecd}, abstractNote={We report on the X-ray emission properties of the pulsar PSR J1849−0001 and its wind nebula (PWN), as measured by Chandra, XMM-Newton, NICER, Swift, and NuSTAR. In the X-ray data, we detected the 38 ms pulsations of the pulsar up to ∼60 keV with high significance. Additionally, we found that the pulsar's on-pulse spectral energy distribution displays significant curvature, peaking at ≈60 keV. Comparing the phase-averaged and on-pulse spectra of the pulsar, we found that the pulsar's off-pulse emission exhibits a spectral shape that is very similar to its on-pulse emission. This characterization of the off-pulse emission enabled us to measure the >10 keV spectrum of the faint and extended PWN using NuSTAR's off-pulse data. We measured both the X-ray spectrum and the radial profiles of the PWN’s brightness and photon index, and we combined these X-ray measurements with published TeV results. We then employed a multizone emission scenario to model the broadband data. The results of the modeling suggest that the magnetic field within the PWN is relatively low (≈7 μG) and that electrons are accelerated to energies ≳400 TeV within this PWN. The electrons responsible for the TeV emission outside the X-ray PWN may propagate to ∼30 pc from the pulsar in ∼10 kyr.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Kim, Chanho and Park, Jaegeun and Woo, Jooyun and Silverman, Sarah and An, Hongjun and Bamba, Aya and Mori, Kaya and Reynolds, Stephen P. and Safi-Harb, Samar}, year={2024}, month={Jan} }
 @article{park_kim_woo_an_mori_reynolds_safi-harb_2023, title={A Broadband X-Ray Study of the Rabbit Pulsar Wind Nebula Powered by PSR J1418-6058}, url={https://doi.org/10.3847/1538-4357/acba0e}, DOI={10.3847/1538-4357/acba0e}, abstractNote={We report on broadband X-ray properties of the Rabbit pulsar wind nebula (PWN) associated with the pulsar PSR J1418−6058 using archival Chandra and XMM-Newton data, as well as a new NuSTAR observation. NuSTAR data above 10 keV allowed us to detect the 110 ms spin period of the pulsar, characterize its hard X-ray pulse profile, and resolve hard X-ray emission from the PWN after removing contamination from the pulsar and other overlapping point sources. The extended PWN was detected up to ∼20 keV and is described well by a power-law model with a photon index Γ ≈ 2. The PWN shape does not vary significantly with energy, and its X-ray spectrum shows no clear evidence of softening away from the pulsar. We modeled the spatial profile of X-ray spectra and broadband spectral energy distribution in the radio to TeV band to infer the physical properties of the PWN. We found that a model with low magnetic field strength (B ∼ 10 μG) and efficient diffusion (D ∼ 1027 cm2 s−1) fits the PWN data well. The extended hard X-ray and TeV emission, associated respectively with synchrotron radiation and inverse Compton scattering by relativistic electrons, suggest that particles are accelerated to very high energies (≳500 TeV), indicating that the Rabbit PWN is a Galactic PeVatron candidate.}, journal={The Astrophysical Journal}, author={Park, Jaegeun and Kim, Chanho and Woo, Jooyun and An, Hongjun and Mori, Kaya and Reynolds, Stephen P. and Safi-Harb, Samar}, year={2023}, month={Mar} }
 @article{guest_borkowski_ghavamian_petre_picquenot_reynolds_seitenzahl_williams_2023, title={Rapid Expansion of the Young Type Ia Supernova Remnant 0519-69.0: More Evidence for a Circumstellar Shell}, volume={946}, ISSN={["1538-4357"]}, url={https://doi.org/10.3847/1538-4357/acbf4e}, DOI={10.3847/1538-4357/acbf4e}, abstractNote={The nature of Type Ia supernovae remains controversial. The youngest remnants of Ia supernovae hold clues to the explosion and to the immediate surroundings. We present a third epoch of Chandra observations of the ∼600 yr old Type Ia remnant 0519–69.0 in the Large Magellanic Cloud, extending the time baseline to 21 yr from the initial 2000 observations. We find rapid expansion of X-ray emitting material, with an average velocity of 4760 km s−1. At the distance of the LMC, this corresponds to an undecelerated age of 750 yr, with the true age somewhat lower. We also find that the bright ring of emission has expanded by 1.3%, corresponding to a velocity of 1900 km s−1 and an undecelerated age of 1600 yr. The high velocity of the peripheral X-rays, contrasted with the modest expansion of the main X-ray shell, provides further evidence for a massive shell of circumstellar material.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Guest, Benson T. and Borkowski, Kazimierz J. and Ghavamian, Parviz and Petre, Robert and Picquenot, Adrien and Reynolds, Stephen P. and Seitenzahl, Ivo R. and Williams, Brian J.}, year={2023}, month={Mar} }
 @article{reynolds_an_abdelmaguid_alford_fryer_mori_nynka_park_terada_woo_et al._2023, title={The High Energy X-ray Probe (HEX-P): supernova remnants, pulsar wind nebulae, and nuclear astrophysics}, volume={10}, ISSN={["2296-987X"]}, DOI={10.3389/fspas.2023.1321278}, abstractNote={HEX-P is a probe-class mission concept that will combine high spatial resolution X-ray imaging (<10″ full width at half maximum) and broad spectral coverage (0.2–80 keV) with an effective area far superior to current facilities (including XMM-Newton and NuSTAR) to enable revolutionary new insights into a variety of important astrophysical problems. HEX-P is ideally suited to address important problems in the physics and astrophysics of supernova remnants (SNRs) and pulsar wind nebulae (PWNe). For shell SNRs, HEX-P can greatly improve our understanding via more accurate spectral characterization and localization of non-thermal X-ray emission from both non-thermal-dominated SNRs and those containing both thermal and non-thermal components, and can discover previously unknown non-thermal components in SNRs. Multi-epoch HEX-P observations of several young SNRs (e.g., Cas A and Tycho) are expected to detect year-scale variabilities of X-ray filaments and knots, thus enabling us to determine fundamental parameters related to diffusive shock acceleration, such as local magnetic field strengths and maximum electron energies. For PWNe, HEX-P will provide spatially-resolved, broadband X-ray spectral data separately from their pulsar emission, allowing us to study how particle acceleration, cooling, and propagation operate in different evolution stages of PWNe. HEX-P is also poised to make unique and significant contributions to nuclear astrophysics of Galactic radioactive sources by improving detections of, or limits on, 44Ti in the youngest SNRs and by potentially discovering rare nuclear lines as evidence of double neutron star mergers. Throughout the paper, we present simulations of each class of objects, demonstrating the power of both the imaging and spectral capabilities of HEX-P to advance our knowledge of SNRs, PWNe, and nuclear astrophysics.}, journal={FRONTIERS IN ASTRONOMY AND SPACE SCIENCES}, author={Reynolds, Stephen and An, Hongjun and Abdelmaguid, Moaz and Alford, Jason and Fryer, Chris and Mori, Kaya and Nynka, Melania and Park, Jaegeun and Terada, Yukikatsu and Woo, Jooyun and et al.}, year={2023}, month={Dec} }
 @article{mori_reynolds_an_bamba_krivonos_tsuji_abdelmaguid_alford_bangale_celli_et al._2023, title={The high energy X-ray probe (HEX-P): Galactic PeVatrons, star clusters, superbubbles, microquasar jets, and gamma-ray binaries}, volume={10}, ISSN={["2296-987X"]}, DOI={10.3389/fspas.2023.1303197}, abstractNote={HEX-P is a probe-class mission concept that will combine high spatial resolution X-ray imaging (<10″ FWHM) and broad spectral coverage (0.2–80 keV) with an effective area far superior to current facilities (including XMM-Newton and NuSTAR) to enable revolutionary new insights into a variety of important astrophysical problems. With the recent discoveries of over 40 ultra-high-energy gamma-ray sources (detected above 100 TeV) and neutrino emission in the Galactic Plane, we have entered a new era of multi-messenger astrophysics facing the exciting reality of Galactic PeVatrons. In the next decade, as more Galactic PeVatrons and TeV gamma-ray sources are expected to be discovered, the identification of their acceleration and emission mechanisms will be the most pressing issue in both particle and high-energy astrophysics. In this paper, along with its companion papers, we will present that HEX-P is uniquely suited to address important problems in various cosmic-ray accelerators, including Galactic PeVatrons, through investigating synchrotron X-ray emission of TeV–PeV electrons produced by both leptonic and hadronic processes. For Galactic PeVatron candidates and other TeV gamma-ray sources, HEX-P can fill in a large gap in the spectral-energy distributions (SEDs) of many objects observed in radio, soft X-rays, and gamma rays, constraining the maximum energies to which electrons can be accelerated, with implications for the nature of the Galactic PeVatrons and their contributions to the spectrum of Galactic cosmic rays beyond the knee at ∼3 PeV. In particular, X-ray observation with HEX-P and TeV observation with CTAO will provide the most powerful multi-messenger diagnostics to identify Galactic PeVatrons and explore a variety of astrophysical shock mechanisms. We present simulations of each class of Galactic TeV–PeV sources, demonstrating the power of both the imaging and spectral capabilities of HEX-P to advance our knowledge of Galactic cosmic-ray accelerators. In addition, we discuss HEX-P’s unique and complementary roles to upcoming gamma-ray and neutrino observatories in the 2030s.}, journal={FRONTIERS IN ASTRONOMY AND SPACE SCIENCES}, author={Mori, Kaya and Reynolds, Stephen and An, Hongjun and Bamba, Aya and Krivonos, Roman and Tsuji, Naomi and Abdelmaguid, Moaz and Alford, Jason and Bangale, Priyadarshini and Celli, Silvia and et al.}, year={2023}, month={Dec} }
 @article{park_kim_woo_an_mori_reynolds_safi-harb_2023, title={X-Ray Studies of the Pulsar PSR J1420–6048 and Its TeV Pulsar Wind Nebula in the Kookaburra Region}, url={https://doi.org/10.3847/1538-4357/acb1b0}, DOI={10.3847/1538-4357/acb1b0}, abstractNote={We present a detailed analysis of broadband X-ray observations of the pulsar PSR J1420−6048 and its wind nebula (PWN) in the Kookaburra region with Chandra, XMM-Newton, and NuSTAR. Using the archival XMM-Newton and new NuSTAR data, we detected 68 ms pulsations of the pulsar and characterized its X-ray pulse profile, which exhibits a sharp spike and a broad bump separated by ∼0.5 in phase. A high-resolution Chandra image revealed a complex morphology of the PWN: a torus-jet structure, a few knots around the torus, one long (∼7′) and two short tails extending in the northwest direction, and a bright diffuse emission region to the south. Spatially integrated Chandra and NuSTAR spectra of the PWN out to 2.′5 are well-described by a power-law model with a photon index Γ ≈ 2. A spatially resolved spectroscopic study, as well as NuSTAR radial profiles of the 3–7 keV and 7–20 keV brightness, showed a hint of spectral softening with increasing distance from the pulsar. A multiwavelength spectral energy distribution (SED) of the source was then obtained by supplementing our X-ray measurements with published radio, Fermi-LAT, and H.E.S.S. data. The SED and radial variations of the X-ray spectrum were fit with a leptonic multizone emission model. Our detailed study of the PWN may be suggestive of (1) particle transport dominated by advection, (2) a low magnetic-field strength (B ∼ 5 μG), and (3) electron acceleration to ∼PeV energies.}, journal={The Astrophysical Journal}, author={Park, Jaegeun and Kim, Chanho and Woo, Jooyun and An, Hongjun and Mori, Kaya and Reynolds, Stephen P. and Safi-Harb, Samar}, year={2023}, month={Mar} }
 @article{guest_borkowski_ghavamian_petre_reynolds_seitenzahl_williams_2022, title={An X-Ray Proper-motion Study of the Large Magellanic Cloud Supernova Remnant 0509-67.5}, volume={164}, ISSN={["1538-3881"]}, url={https://doi.org/10.3847/1538-3881/ac9792}, DOI={10.3847/1538-3881/ac9792}, abstractNote={We present a third epoch of Chandra observations of the Type Ia Large Magellanic Cloud Supernova remnant 0509-67.5. With these new observations from 2020, the baseline for proper-motion measurements of the expansion has grown to 20 yr (from the earliest Chandra observations in 2000). We report here the results of these new expansion measurements. The lack of nearby bright point sources renders absolute image alignment difficult. However, we are able to measure the average expansion of the diameter of the remnant along several projection directions. We find that the remnant is expanding with an average velocity of 6120 (4900–7360) km s−1. This high shock velocity is consistent with previous works, and also consistent with the inference that 0509-67.5 is expanding into a very low density surrounding medium. At the distance of the LMC, this velocity corresponds to an undecelerated age of 600 yr, with the real age somewhat smaller.}, number={6}, journal={ASTRONOMICAL JOURNAL}, author={Guest, Benson T. and Borkowski, Kazimierz J. and Ghavamian, Parviz and Petre, Robert and Reynolds, Stephen P. and Seitenzahl, Ivo R. and Williams, Brian J.}, year={2022}, month={Dec} }
 @article{williams_ghavamian_seitenzahl_reynolds_borkowski_petre_2022, title={Evidence for a Dense, Inhomogeneous Circumstellar Medium in the Type Ia SNR 0519-69.0}, volume={935}, ISSN={["1538-4357"]}, url={https://doi.org/10.3847/1538-4357/ac81ca}, DOI={10.3847/1538-4357/ac81ca}, abstractNote={We perform an expansion study of the Balmer-dominated outer shock of the SNR 0519−69.0 in the LMC by using a combination of new Hubble Space Telescope (HST) WFC3 imagery obtained in 2020 and archival ACS images from 2010 and 2011. Thanks to the very long time baseline, our proper motion measurements are of unprecedented accuracy. We find a wide range of shock velocities, with the fastest shocks averaging 5280 km s−1 and the slowest grouping of shocks averaging just 1670 km s−1. We compare the Hα images from HST with X-ray images from Chandra and mid-IR images from Spitzer, finding a clear anticorrelation between the brightness of the remnant in a particular location and the velocity of the blast wave at that location, supporting the idea that the bright knots of X-ray and IR emission result from an interaction with a dense inhomogeneous circumstellar medium. We find no evidence for X-ray emission, thermal or nonthermal, associated with the fastest shocks, as expected if the fastest velocities are the result of the blast wave encountering the lower density ambient medium of the LMC. We derive an age of the remnant of ≤670 ± 70 yr, consistent with results derived from previous investigations.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Williams, Brian J. and Ghavamian, Parviz and Seitenzahl, Ivo R. and Reynolds, Stephen P. and Borkowski, Kazimierz J. and Petre, Robert}, year={2022}, month={Aug} }
 @article{burgess_mori_gelfand_hailey_tokayer_woo_an_malone_reynolds_safi-harb_et al._2022, title={The Eel Pulsar Wind Nebula: A PeVatron-candidate Origin for HAWC J1826-128 and HESS J1826-130}, volume={930}, ISSN={["1538-4357"]}, url={https://doi.org/10.3847/1538-4357/ac650a}, DOI={10.3847/1538-4357/ac650a}, abstractNote={HAWC J1826−128 is one of the brightest Galactic TeV γ-ray sources detected by the High Altitude Water Cherenkov (HAWC) observatory, with photon energies extending up to nearly ∼100 TeV. This HAWC source spatially coincides with the H.E.S.S. TeV source HESS J1826−130 and the “Eel” pulsar wind nebula (PWN), which is associated with the GeV pulsar PSR J1826−1256. In the X-ray band, Chandra and XMM-Newton revealed that the Eel PWN is composed of both a compact nebula (∼15″) and diffuse X-ray emission (∼6′ × 2′) extending away from the pulsar. Our NuSTAR observation detected hard X-ray emission from the compact PWN up to ∼20 keV and evidence of the synchrotron burn-off effect. In addition to the spatial coincidence between HESS J1826−130 and the diffuse X-ray PWN, our multiwavelength spectral energy distribution (SED) analysis using X-ray and γ-ray data establishes a leptonic origin of the TeV emission associated with the Eel PWN. Furthermore, our evolutionary PWN SED model suggests (1) a low PWN B-field of ∼1 μG, (2) a significantly younger pulsar age (t ∼ 5.7 kyr) than the characteristic age (τ = 14.4 kyr), and (3) a maximum electron energy of Emax=2 PeV. The low B-field, as well as the putative supersonic motion of the pulsar, may account for the asymmetric morphology of the diffuse X-ray emission. Our results suggest that the Eel PWN may be a leptonic PeVatron particle accelerator powered by the ∼6 kyr old pulsar PSR J1826−1256 with a spin-down power of 3.6 × 1036 erg s−1.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Burgess, Daniel A. and Mori, Kaya and Gelfand, Joseph D. and Hailey, Charles J. and Tokayer, Yarone M. and Woo, Jooyun and An, Hongjun and Malone, Kelly and Reynolds, Stephen P. and Safi-Harb, Samar and et al.}, year={2022}, month={May} }
 @article{reynolds_williams_borkowski_long_2021, title={Efficiencies of Magnetic Field Amplification and Electron Acceleration in Young Supernova Remnants: Global Averages and Kepler's Supernova Remnant}, volume={917}, ISSN={["1538-4357"]}, url={https://doi.org/10.3847/1538-4357/ac0ced}, DOI={10.3847/1538-4357/ac0ced}, abstractNote={Particle acceleration to suprathermal energies in strong astrophysical shock waves is a widespread phenomenon, generally explained by diffusive shock acceleration. Such shocks can also amplify the upstream magnetic field considerably beyond simple compression. The complex plasma physics processes involved are often parameterized by assuming that shocks put some fraction ϵ e of their energy into fast particles and another fraction ϵ B into the magnetic field. Modelers of shocks in supernovae, supernova remnants (SNRs), and gamma-ray bursters, among other locations, often assume typical values for these fractions, presumed to remain constant in time. However, it is rare that enough properties of a source are independently constrained that values of the epsilons can be inferred directly. SNRs can provide such circumstances. Here we summarize results from global fits to spatially integrated emission in six young SNRs, finding 10−4 ≲ ϵ e ≲ 0.05 and 0.001 ≲ ϵ B ≲ 0.1. These large variations might be put down to the differing ages and environments of these SNRs, so we conduct a detailed analysis of a single remnant, that of Kepler’s supernova. Both epsilons can be determined at seven different locations around the shock, and we find even larger ranges for both epsilons, as well as for their ratio (thus independent of the shock energy itself). We conclude that unknown factors have a large influence on the efficiency of both processes. Shock obliquity, upstream neutral fraction, or other possibilities need to be explored, while calculations assuming fixed values of the epsilons should be regarded as provisional.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Reynolds, Stephen P. and Williams, Brian J. and Borkowski, Kazimierz J. and Long, Knox S.}, year={2021}, month={Aug} }
 @article{stone_johnson_blondin_watson_borkowski_frohlich_seitenzahl_reynolds_2021, title={Type Ia Supernova Models: Asymmetric Remnants and Supernova Remnant G1.9+0.3}, volume={923}, ISSN={["1538-4357"]}, url={https://doi.org/10.3847/1538-4357/ac300f}, DOI={10.3847/1538-4357/ac300f}, abstractNote={The youngest Galactic supernova remnant, G1.9+0.3, probably the result of a Type Ia supernova, shows surprising anomalies in the distribution of its ejecta in space and velocity. In particular, high-velocity shocked iron is seen in several locations far from the remnant center, in some cases beyond prominent silicon and sulfur emission. These asymmetries strongly suggest a highly asymmetric explosion. We present high-resolution hydrodynamic simulations in two and three dimensions of the evolution from ages of 100 s to hundreds of years of two asymmetric Type Ia models, expanding into a uniform medium. At the age of G1.9+0.3 (about 100 yr), our 2D model shows almost no iron shocked to become visible in X-rays. Only in a much higher-density environment could significant iron be shocked, at which time the model's expansion speed is completely inconsistent with the observations of G1.9+0.3. Our 3D model, evolving the most asymmetric of a suite of Type Ia supernova models from Seitenzahl et al. (2013), shows some features resembling G1.9+0.3. We characterize its evolution with images of composition in three classes: C and O, intermediate-mass elements (IMEs), and iron-group elements (IGEs). From ages of 13 to 1800 yr, we follow the evolution of the highly asymmetric initial remnant as the explosion asymmetries decrease in relative strength, to be replaced by asymmetries due to evolutionary hydrodynamic instabilities. At an age of about 100 yr, our 3D model has comparable shocked masses of C+O, IMEs, and IGEs, with about 0.03 M ⊙ each. Evolutionary changes appear to be rapid enough that continued monitoring with the Chandra X-ray Observatory may show significant variations.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, publisher={American Astronomical Society}, author={Stone, Alice G. and Johnson, Heather T. and Blondin, John M. and Watson, Richard A. and Borkowski, Kazimierz J. and Frohlich, Carla and Seitenzahl, Ivo R. and Reynolds, Stephen P.}, year={2021}, month={Dec} }
 @article{borkowski_miltich_reynolds_2020, title={Expansion and Age of the Supernova Remnant G350.1–0.3: High-velocity Iron Ejecta from a Core-collapse Event}, url={https://doi.org/10.3847/2041-8213/abcda7}, DOI={10.3847/2041-8213/abcda7}, abstractNote={We report Chandra observations of the highly asymmetric core-collapse supernova remnant G350.1−0.3. We document expansion over 9 yr away from the roughly stationary central compact object, with sky-plane velocities up to 5000 d4.5 km s−1 (d4.5 is the distance in units of 4.5 kpc), redshifts ranging from 900 to 2600 km s−1, and three-dimensional space velocities approaching 6000 km s−1. Most of the bright emission comes from heavy-element ejecta particularly strong in iron. Iron-enhanced ejecta are seen at 4000–6000 km s−1, strongly suggesting that the supernova was not a common Type IIP event. While some fainter regions have roughly solar abundances, we cannot identify clear blast-wave features. Our expansion proper motions indicate that G350.1−0.3 is no more than about 600 yr old, independent of distance: the third youngest known core-collapse supernova in the Galaxy, and one of the most asymmetric.}, journal={The Astrophysical Journal Letters}, author={Borkowski, Kazimierz J. and Miltich, William and Reynolds, Stephen P.}, year={2020}, month={Dec} }
 @article{borkowski_reynolds_miltich_2020, title={Fast Blast Wave and Ejecta in the Young Core-collapse Supernova Remnant MSH 15-52/RCW 89}, url={https://doi.org/10.3847/2041-8213/ab91c0}, DOI={10.3847/2041-8213/ab91c0}, abstractNote={One of the youngest known remnants of a core-collapse supernova (SN) in our Galaxy is G320.4−1.2/MSH 15-52, containing an energetic pulsar with a very short (1700 yr) spindown age and likely produced by a stripped-envelope SN Ibc. Bright X-ray and radio emission north of the pulsar overlaps with an Hα nebula RCW 89. The bright X-rays there have a highly unusual and quite puzzling morphology, consisting of both very compact thermally emitting knots and much more diffuse emission of nonthermal origin. We report new X-ray observations of RCW 89 in 2017 and 2018 with Chandra that allowed us to measure the motions of many knots and filaments on decade-long time baselines. We identify a fast blast wave with a velocity of km s−1 ( is the distance in units of 5.2 kpc) with a purely nonthermal spectrum, and without any radio counterpart. Many compact X-ray emission knots are moving vary fast, with velocities as high as 5000 km s−1, predominantly radially away from the pulsar. Their spectra show that they are Ne- and Mg-rich heavy-element SN ejecta. They have been significantly decelerated upon their recent impact with the dense ambient medium north of the pulsar. We see fast evolution in brightness and morphology of knots in just a few years. Ejecta knots in RCW 89 resemble those seen in Cas A at optical wavelengths in terms of their initial velocities and densities. They might have the same origin, still not understood but presumably related to stripped-envelope SN explosions themselves.}, journal={The Astrophysical Journal Letters}, author={Borkowski, Kazimierz J. and Reynolds, Stephen P. and Miltich, William}, year={2020}, month={Jun} }
 @article{madsen_fryer_grefenstette_lopez_reynolds_zoglauer_2020, title={NuSTAR Observations of G11.2–0.3}, url={https://doi.org/10.3847/1538-4357/ab54ca}, DOI={10.3847/1538-4357/ab54ca}, abstractNote={We present in this paper the hard X-ray view of the pulsar wind nebula in G11.2−0.3 and its central pulsar powered pulsar J1811−1925 as seen by NuSTAR. We complement the data with Chandra for a more complete picture and confirm the existence of a hard, power-law component in the shell with photon index , which we attribute to synchrotron emission. Our imaging observations of the shell show a slightly smaller radius at higher energies, consistent with Chandra results, and we find shrinkage as a function of increased energy along the jet direction, indicating that the electron outflow in the PWN may be simpler than that seen in other young PWNe. Combining NuSTAR with INTEGRAL, we find that the pulsar spectrum can be fit by a power law with up to 300 keV without evidence of curvature.}, journal={The Astrophysical Journal}, author={Madsen, K. K. and Fryer, C. L. and Grefenstette, B. W. and Lopez, L. A. and Reynolds, S. and Zoglauer, A.}, year={2020}, month={Jan} }
 @article{reynolds_borkowski_2019, title={On the Expansion, Age, and Origin of the Puzzling Shell/Pulsar-wind Nebula G310.6–1.6}, url={https://doi.org/10.3847/1538-4357/ab5804}, DOI={10.3847/1538-4357/ab5804}, abstractNote={We present a 142 ks Chandra observation of the enigmatic combination supernova remnant G310.6–1.6 consisting of a bright pulsar-wind nebula driven by an energetic pulsar, surrounded by a highly circular, very faint shell with a featureless, probably synchrotron, spectrum. Comparison with an observation 6 yr earlier shows no measurable expansion of the shell, though some features in the pulsar-wind nebula have moved. We find an expansion age of at least 2500 yr, implying a current shock velocity less than about 1000 km s−1. We place severe upper limits on thermal emission from the shell; if the shell locates the blast wave, a Sedov interpretation would require the remnant to be very young, about 1000 yr, and to have resulted from a dramatically sub-energetic supernova, ejecting ≪0.02M⊙ with energy E ≲ 3 × 1047 erg. Even a merger-induced collapse of a white dwarf to a neutron star, with a low-energy explosion, is unlikely to produce such an event. Other explanations seem equally unlikely.}, journal={The Astrophysical Journal}, author={Reynolds, Stephen P. and Borkowski, Kazimierz J.}, year={2019}, month={Dec} }
 @article{borkowski_reynolds_williams_petre_2018, title={Expansion and Age of the X-Ray Synchrotron-dominated Supernova Remnant G330.2+1.0}, volume={868}, ISSN={["2041-8213"]}, url={https://doi.org/10.3847/2041-8213/aaedb5}, DOI={10.3847/2041-8213/aaedb5}, abstractNote={We report new Chandra observations of one of the few Galactic supernova remnants whose X-ray spectrum is dominated by nonthermal synchrotron radiation, G330.2+1.0. We find that between 2006 and 2017, some parts of the shell have expanded by about 1%, giving a free-expansion (undecelerated) age of about 1000 yr, and implying shock velocities there of 9000 km s−1 for a distance of 5 kpc. Somewhat slower expansion is seen elsewhere around the remnant periphery, in particular in compact knots. Because some deceleration must have taken place, we infer that G330.2+1.0 is less than about 1000 yr old. Thus, G330.2+1.0 is one of only four Galactic core-collapse remnants of the last millennium. The large size, low brightness, and young age require a very low ambient density, suggesting expansion in a stellar-wind bubble. We suggest that in the east, where some thermal emission is seen and expansion velocities are much slower, the shock has reached the edge of the cavity. The high shock velocities can easily accelerate relativistic electrons to X-ray-emitting energies. A few small regions show highly significant brightness changes by 10%–20%, both brightening and fading, a phenomenon previously observed in only two supernova remnants, indicating strong and/or turbulent magnetic fields.}, number={2}, journal={ASTROPHYSICAL JOURNAL LETTERS}, author={Borkowski, Kazimierz J. and Reynolds, Stephen P. and Williams, Brian J. and Petre, Robert}, year={2018}, month={Dec} }
 @article{reynolds_borkowski_gwynne_2018, title={Expansion and Brightness Changes in the Pulsar-wind Nebula in the Composite Supernova Remnant Kes 75}, volume={856}, ISSN={["1538-4357"]}, url={https://doi.org/10.3847/1538-4357/aab3d3}, DOI={10.3847/1538-4357/aab3d3}, abstractNote={We report new Chandra X-ray observations of the shell supernova remnant Kes 75 (G29.7−0.3) containing a pulsar and pulsar-wind nebula (PWN). Expansion of the PWN is apparent across four epochs—2000, 2006, 2009, and 2016. We find an expansion rate between 2000 and 2016 of the northwest edge of the PWN of 0.249% ± 0.023% yr−1, for an expansion age R/(dR/dt) of 400 ± 40 yr and an expansion velocity of about 1000 km s−1. We suggest that the PWN is expanding into an asymmetric nickel bubble in a conventional Type IIP supernova. Some acceleration of the PWN expansion is likely, giving a true age of 480 ± 50 yr. The pulsar’s birth luminosity was larger than the current value by a factor of 3–8, while the initial period was within a factor of 2 of its current value. We confirm directly that Kes 75 contains the youngest known PWN, and hence the youngest known pulsar. The pulsar PSR J1846−0258 has a spindown-inferred magnetic field of 5 × 1013 G; in 2006, it emitted five magnetar-like short X-ray bursts, but its spindown luminosity has not changed significantly. However, the flux of the PWN has decreased by about 10% between 2009 and 2016, almost entirely in the northern half. A bright knot has declined by 30% since 2006. During this time, the photon indices of the power-law models did not change. This flux change is too rapid to be due to normal PWN evolution in one-zone models.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Reynolds, Stephen P. and Borkowski, Kazimierz J. and Gwynne, Peter H.}, year={2018}, month={Apr} }
 @article{williams_blair_borkowski_ghavamian_hendrick_long_petre_raymond_rest_reynolds_et al._2018, title={The Expansion of the Young Supernova Remnant 0509-68.7 (N103B)}, volume={865}, ISSN={["2041-8213"]}, url={https://doi.org/10.3847/2041-8213/aae08d}, DOI={10.3847/2041-8213/aae08d}, abstractNote={We present a second epoch of Chandra observations of the Type Ia Large Magellanic Cloud supernova remnant (SNR) 0509-68.7 (N103B) obtained in 2017. When combined with the earlier observations from 1999, we have a 17.4 year baseline with which we can search for evidence of the remnant’s expansion. Although the lack of strong point source detections makes absolute image alignment at the necessary accuracy impossible, we can measure the change in the diameter and the area of the remnant, and find that it has expanded by an average velocity of 4170 (2860, 5450) km s−1. This supports the picture of this being a young remnant; this expansion velocity corresponds to an undecelerated age of 850 years, making the real age somewhat younger, consistent with results from light echo studies. Previous infrared observations have revealed high densities in the western half of the remnant, likely from circumstellar material, so it is probable that the real expansion velocity is lower on that side of the remnant and higher on the eastern side. A similar scenario is seen in Kepler’s SNR. N103B joins the rare class of Magellanic Cloud SNRs with measured proper motions.}, number={2}, journal={ASTROPHYSICAL JOURNAL LETTERS}, author={Williams, Brian J. and Blair, William P. and Borkowski, Kazimierz J. and Ghavamian, Parviz and Hendrick, Sean P. and Long, Knox S. and Petre, Robert and Raymond, John C. and Rest, Armin and Reynolds, Stephen P. and et al.}, year={2018}, month={Oct} }
 @article{temim_dwek_arendt_borkowski_reynolds_slane_gelfand_raymond_2017, title={A Massive Shell of Supernova-formed Dust in SNR G54.1+0.3}, volume={836}, ISSN={["1538-4357"]}, url={https://doi.org/10.3847/1538-4357/836/1/129}, DOI={10.3847/1538-4357/836/1/129}, abstractNote={While theoretical models of dust condensation predict that most refractory elements produced in core-collapse supernovae (SNe) efficiently condense into dust, a large quantity of dust has so far only been observed in SN 1987A. We present an analysis of observations from the Spitzer Space Telescope, Herschel Space Observatory, Stratospheric Observatory for Infrared Astronomy, and AKARI of the infrared shell surrounding the pulsar wind nebula in the supernova remnant G54.1+0.3. We attribute a distinctive spectral feature at 21 μm to a magnesium silicate grain species that has been invoked in modeling the ejecta-condensed dust in Cas A, which exhibits the same spectral signature. If this species is responsible for producing the observed spectral feature and accounts for a significant fraction of the observed infrared continuum, we find that it would be the dominant constituent of the dust in G54.1+0.3, with possible secondary contributions from other compositions, such as carbon, silicate, or alumina grains. The total mass of SN-formed dust required by this model is at least 0.3 M⊙. We discuss how these results may be affected by varying dust grain properties and self-consistent grain heating models. The spatial distribution of the dust mass and temperature in G54.1+0.3 confirms the scenario in which the SN-formed dust has not yet been processed by the SN reverse shock and is being heated by stars belonging to a cluster in which the SN progenitor exploded. The dust mass and composition suggest a progenitor mass of 16–27 M⊙ and imply a high dust condensation efficiency, similar to that found for Cas A and SN 1987A. The study provides another example of significant dust formation in a Type IIP SN explosion and sheds light on the properties of pristine SN-condensed dust.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Temim, Tea and Dwek, Eli and Arendt, Richard G. and Borkowski, Kazimierz J. and Reynolds, Stephen P. and Slane, Patrick and Gelfand, Joseph D. and Raymond, John C.}, year={2017}, month={Feb} }
 @article{borkowski_gwynne_reynolds_green_hwang_petre_willett_2017, title={Asymmetric Expansion of the Youngest Galactic Supernova Remnant G1.9+0.3}, volume={837}, url={https://doi.org/10.3847/2041-8213/aa618c}, DOI={10.3847/2041-8213/aa618c}, abstractNote={The youngest Galactic supernova remnant (SNR) G1.9+0.3, produced by a (probable) SN Ia that exploded ∼1900 CE, is strongly asymmetric at radio wavelengths, much brighter in the north, but bilaterally symmetric in X-rays. We present the results of X-ray expansion measurements that illuminate the origin of the radio asymmetry. We confirm the mean expansion rate (2011–2015) of 0.58% yr−1, but large spatial variations are present. Using the nonparametric “Demons” method, we measure the velocity field throughout the entire SNR, finding that motions vary by a factor of 5, from to yr−1. The slowest shocks are at the outer boundary of the bright northern radio rim, with velocities vs as low as 3600 km s−1 (for an assumed distance of 8.5 kpc), much less than vs = 12,000–13,000 km s−1 along the X-ray-bright major axis. Such strong deceleration of the northern blast wave most likely arises from the collision of SN ejecta with a much denser than average ambient medium there. This asymmetric ambient medium naturally explains the radio asymmetry. In several locations, significant morphological changes and strongly nonradial motions are apparent. The spatially integrated X-ray flux continues to increase with time. Based on Chandra observations spanning 8.3 yr, we measure its increase at yr−1. The SN ejecta are likely colliding with the asymmetric circumstellar medium ejected by the SN progenitor prior to its explosion.}, number={1}, journal={The Astrophysical Journal Letters}, author={Borkowski, Kazimierz J. and Gwynne, Peter and Reynolds, Stephen P. and Green, David A. and Hwang, Una and Petre, Robert and Willett, Rebecca}, year={2017}, month={Mar} }
 @article{borkowski_reynolds_2017, title={Expansion of Kes 73, A Shell Supernova Remnant Containing a Magnetar}, volume={846}, ISSN={["1538-4357"]}, DOI={10.3847/1538-4357/aa830f}, abstractNote={Of the 30 or so Galactic magnetars, about 8 are in supernova remnants (SNRs). One of the most extreme magnetars, 1E 1841−045, is at the center of the SNR Kes 73 (G27.4+0.0), whose age is uncertain. We measure its expansion using three Chandra observations over 15 years, obtaining a mean rate of yr−1. For a distance of 8.5 kpc, we obtain a shell velocity of 1100 km s−1 and infer a blast wave speed of 1400 km s−1. For Sedov expansion into a uniform medium, this gives an age of 1800 years. Derived emission measures imply an ambient density of about 2 cm−3 and an upper limit on the swept-up mass of about , with lower limits of tens of , confirming that Kes 73 is in an advanced evolutionary stage. Our spectral analysis shows no evidence for enhanced abundances as would be expected from a massive progenitor. Our derived total energy is erg, giving a very conservative lower limit to the magnetar’s initial period of about 3 ms, unless its energy was lost by non-electromagnetic means. We see no evidence of a wind-blown bubble as would be produced by a massive progenitor, or any evidence that the progenitor of Kes 73/1E 1841−045 was anything but a normal red supergiant producing a Type IIP supernova, though a short-lived stripped-envelope progenitor cannot be absolutely excluded. Kes 73's magnetar thus joins SGR 1900+14 as magnetars resulting from relatively low-mass progenitors.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Borkowski, Kazimierz J. and Reynolds, Stephen P.}, year={2017}, month={Sep} }
 @article{reynolds_pavlov_kargaltsev_klingler_renaud_mereghetti_2017, title={Pulsar-Wind Nebulae and Magnetar Outflows: Observations at Radio, X-Ray, and Gamma-Ray Wavelengths}, volume={207}, ISSN={0038-6308 1572-9672}, url={http://dx.doi.org/10.1007/S11214-017-0356-6}, DOI={10.1007/S11214-017-0356-6}, abstractNote={We review observations of several classes of neutron-star-powered outflows: pulsar-wind nebulae (PWNe) inside shell supernova remnants (SNRs), PWNe interacting directly with interstellar medium (ISM), and magnetar-powered outflows. We describe radio, X-ray, and gamma-ray observations of PWNe, focusing first on integrated spectral-energy distributions (SEDs) and global spectral properties. High-resolution X-ray imaging of PWNe shows a bewildering array of morphologies, with jets, trails, and other structures. Several of the 23 so far identified magnetars show evidence for continuous or sporadic emission of material, sometimes associated with giant flares, and a few possible "magnetar-wind nebulae" have been recently identified.}, number={1-4}, journal={Space Science Reviews}, publisher={Springer Science and Business Media LLC}, author={Reynolds, Stephen P. and Pavlov, George G. and Kargaltsev, Oleg and Klingler, Noel and Renaud, Matthieu and Mereghetti, Sandro}, year={2017}, month={Mar}, pages={175–234} }
 @article{reynolds_pavlov_kargaltsev_klingler_renaud_mereghetti_2017, title={Pulsar-wind nebulae and magnetar outflows: Observations at radio, x-ray, and gamma-ray wavelengths}, volume={207}, DOI={10.1007/978-94-024-1292-5_7}, number={1-4}, journal={Space Science Reviews}, author={Reynolds, Stephen and Pavlov, G. G. and Kargaltsev, O. and Klingler, N. and Renaud, M. and Mereghetti, S.}, year={2017}, pages={175–234} }
 @article{grefenstette_fryer_harrison_boggs_delaney_laming_reynolds_alexander_barret_christensen_et al._2017, title={THE DISTRIBUTION OF RADIOACTIVE 44Ti IN CASSIOPEIA A}, url={https://doi.org/10.3847/1538-4357/834/1/19}, DOI={10.3847/1538-4357/834/1/19}, abstractNote={The distribution of elements produced in the innermost layers of a supernova explosion is a key diagnostic for studying the collapse of massive stars. Here we present the results of a 2.4 Ms NuSTAR observing campaign aimed at studying the supernova remnant Cassiopeia A (Cas A). We perform spatially resolved spectroscopic analyses of the 44Ti ejecta, which we use to determine the Doppler shift and thus the three-dimensional (3D) velocities of the 44Ti ejecta. We find an initial 44Ti mass of (1.54 ± 0.21) × 10−4 M⊙, which has a present-day average momentum direction of 340° ± 15° projected onto the plane of the sky (measured clockwise from celestial north) and is tilted by 58° ± 20° into the plane of the sky away from the observer, roughly opposite to the inferred direction of motion of the central compact object. We find some 44Ti ejecta that are clearly interior to the reverse shock and some that are clearly exterior to it. Where we observe 44Ti ejecta exterior to the reverse shock we also see shock-heated iron; however, there are regions where we see iron but do not observe 44Ti. This suggests that the local conditions of the supernova shock during explosive nucleosynthesis varied enough to suppress the production of 44Ti by at least a factor of two in some regions, even in regions that are assumed to be the result of processes like α-rich freezeout that should produce both iron and titanium.}, journal={The Astrophysical Journal}, author={Grefenstette, Brian W. and Fryer, Chris L. and Harrison, Fiona A. and Boggs, Steven E. and DeLaney, Tracey and Laming, J. Martin and Reynolds, Stephen P. and Alexander, David M. and Barret, Didier and Christensen, Finn E. and et al.}, year={2017}, month={Jan} }
 @article{williams_coyle_yamaguchi_depasquale_seitenzahl_hewitt_blondin_borkowski_ghavamian_petre_et al._2017, title={The Three-dimensional Expansion of the Ejecta from Tycho's Supernova Remnant}, volume={842}, ISSN={["1538-4357"]}, url={https://doi.org/10.3847/1538-4357/aa7384}, DOI={10.3847/1538-4357/aa7384}, abstractNote={We present the first 3D measurements of the velocity of various ejecta knots in Tycho’s supernova remnant, known to result from a Type Ia explosion. Chandra X-ray observations over a 12 yr baseline from 2003 to 2015 allow us to measure the proper motion of nearly 60 “tufts” of Si-rich ejecta, giving us the velocity in the plane of the sky. For the line-of-sight velocity, we use two different methods: a nonequilibrium ionization model fit to the strong Si and S lines in the 1.2–2.8 keV regime, and a fit consisting of a series of Gaussian lines. These methods give consistent results, allowing us to determine the redshift or blueshift of each of the knots. Assuming a distance of 3.5 kpc, we find total velocities that range from 2400 to 6600 km s−1, with a mean of 4430 km s−1. We find several regions where the ejecta knots have overtaken the forward shock. These regions have proper motions in excess of 6000 km s−1. Some SN Ia explosion models predict a velocity asymmetry in the ejecta. We find no such velocity asymmetries in Tycho, and we discuss our findings in light of various explosion models, favoring those delayed-detonation models with relatively vigorous and symmetrical deflagrations. Finally, we compare measurements with models of the remnant’s evolution that include both smooth and clumpy ejecta profiles, finding that both ejecta profiles can be accommodated by the observations.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Williams, Brian J. and Coyle, Nina M. and Yamaguchi, Hiroya and Depasquale, Joseph and Seitenzahl, Ivo R. and Hewitt, John W. and Blondin, John M. and Borkowski, Kazimierz J. and Ghavamian, Parviz and Petre, Robert and et al.}, year={2017}, month={Jun} }
 @article{reynolds_borkowski_2016, title={A COMPACT X-RAY SOURCE IN THE RADIO PULSAR-WIND NEBULA G141.2+5.0}, volume={816}, ISSN={["2041-8213"]}, url={https://doi.org/10.3847/2041-8205/816/2/L27}, DOI={10.3847/2041-8205/816/2/l27}, abstractNote={We report the results of a 50 ks Chandra observation of the recently discovered radio object G141.2+5.0, presumed to be a pulsar-wind nebula. We find a moderately bright unresolved X-ray source that we designate CXOU J033712.8 615302 coincident with the central peak radio emission. An absorbed power-law fit to the 241 counts describes the data well, with absorbing column N H = 6.7 ( 4.0 , 9.7 ) × 10 21 ?> cm−2 and photon index Γ = 1.8 ( 1.4 , 2.2 ) ?> . For a distance of 4 kpc, the unabsorbed luminosity between 0.5 and 8 keV is 1.7 − 0.3 + 0.4 × 10 32 ?> erg s−1 (90% confidence intervals). Both LX and Γ are quite typical of pulsars in PWNe. No extended emission is seen; we estimate a conservative 3 &sgr; ?> upper limit to the surface brightness of any X-ray PWN near the point source to be 3 × 10 − 17 ?> erg cm−2 s−1 arcsec−2 between 0.5 and 8 keV, assuming the same spectrum as the point source; for a nebula of diameter 13 ″ ?> , the flux limit is 6% of the flux of the point source. The steep radio spectrum of the PWN ( &agr; ∼ − 0.7 ?> ), if continued to the X-ray without a break, predicts L X (nebula) ∼ 1 × 10 33 ?> erg s−1, so additional spectral steepening between radio and X-rays is required, as is true of all known PWNe. The high Galactic latitude gives a z-distance of 350 pc above the Galactic plane, quite unusual for a Population I object.}, number={2}, journal={ASTROPHYSICAL JOURNAL LETTERS}, author={Reynolds, Stephen P. and Borkowski, Kazimierz J.}, year={2016}, month={Jan} }
 @article{williams_chomiuk_hewitt_blondin_borkowski_ghavamian_petre_reynolds_2016, title={AN X-RAY AND RADIO STUDY OF THE VARYING EXPANSION VELOCITIES IN TYCHO’S SUPERNOVA REMNANT}, volume={823}, ISSN={2041-8213}, url={http://dx.doi.org/10.3847/2041-8205/823/2/L32}, DOI={10.3847/2041-8205/823/2/l32}, abstractNote={We present newly obtained X-ray and radio observations of Tycho’s supernova remnant using Chandra and the Karl G. Jansky Very Large Array in 2015 and 2013/14, respectively. When combined with earlier epoch observations by these instruments, we now have time baselines for expansion measurements of the remnant of 12–15 years in the X-rays and 30 years in the radio. The remnant’s large angular size allows for proper motion measurements at many locations around the periphery of the blast wave. Consistent with earlier measurements, we find a clear gradient in the expansion velocity of the remnant, despite its round shape. The proper motions on the western and southwestern sides of the remnant are about a factor of two higher than those in the east and northeast. We showed in an earlier work that this is related to an offset of the explosion site from the geometric center of the remnant due to a density gradient in the ISM, and using our refined measurements reported here, we find that this offset is ∼23″ toward the northeast. An explosion center offset in such a circular remnant has implications for searches for progenitor companions in other remnants.}, number={2}, journal={The Astrophysical Journal}, publisher={American Astronomical Society}, author={Williams, Brian J. and Chomiuk, Laura and Hewitt, John W. and Blondin, John M. and Borkowski, Kazimierz J. and Ghavamian, Parviz and Petre, Robert and Reynolds, Stephen P.}, year={2016}, month={May}, pages={L32} }
 @article{borkowski_reynolds_roberts_2016, title={G11.2-0.3: THE YOUNG REMNANT OF A STRIPPED-ENVELOPE SUPERNOVA}, volume={819}, ISSN={["1538-4357"]}, DOI={10.3847/0004-637x/819/2/160}, abstractNote={We present results of a 400 ks Chandra observation of the young shell supernova remnant (SNR) G11.2−0.3, containing a pulsar and pulsar-wind nebula (PWN). We measure a mean expansion rate for the shell since 2000 of 0.0277 ± 0.0018% yr−1, implying an age between 1400 and 2400 yr, and making G11.2−0.3 one of the youngest core-collapse SNRs in the Galaxy. However, we find very high absorption (AV ∼ 16m ± 2m), confirming near-IR determinations and ruling out a claimed association with the possible historical SN of 386 CE. The PWN shows strong jets and a faint torus within a larger, more diffuse region of radio emission and nonthermal X-rays. Central soft thermal X-ray emission is anticorrelated with the PWN; that, and more detailed morphological evidence, indicates that the reverse shock has already reheated all ejecta and compressed the PWN. The pulsar characteristic energy-loss timescale is well in excess of the remnant age, and we suggest that the bright jets have been produced since the recompression. The relatively pronounced shell and diffuse hard X-ray emission in the interior, enhanced at the inner edge of the shell, indicate that the immediate circumstellar medium into which G11.2−0.3 is expanding was quite anisotropic. We propose a possible origin for G11.2−0.3 in a stripped-envelope progenitor that had lost almost all its envelope mass, in an anisotropic wind or due to binary interaction, leaving a compact core whose fast winds swept previously lost mass into a dense irregular shell, and which exploded as a SN cIIb or Ibc.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Borkowski, Kazimierz J. and Reynolds, Stephen P. and Roberts, Mallory S. E.}, year={2016}, month={Mar} }
 @article{reynoldst_2016, title={Hard X-ray emission from pulsar-wind nebulae}, volume={82}, ISSN={["1469-7807"]}, url={https://doi.org/10.1017/S0022377816000751}, DOI={10.1017/s0022377816000751}, abstractNote={Pulsar-wind nebulae emit an extremely broad spectrum of continuum radiation, from low radio frequencies to TeV gamma rays. The part of the spectral energy distribution (SED) from radio through MeV gamma rays is due to synchrotron emission from a distribution of relativistic electrons (or pairs) which can be described by one or more power laws. This spectrum exhibits that particle energy distribution, responsible also for the higher-energy (GeV–TeV) part of the SED, due to inverse-Compton upscattering of one of three photon fields: the synchrotron spectrum, the cosmic microwave background, or ambient optical/infrared photons. However, in a few sources, primary hadrons may produce GeV–TeV gamma rays through the decay of neutral pions produced in inelastic cosmic-ray collisions with thermal gas. The higher-energy end of the particle spectrum, producing synchrotron photons above approximately 10 keV, holds clues to the particle acceleration process. However, its detailed study requires imaging spectroscopy in this energy range, not available until the NuSTAR mission beginning in 2012, which performs true imaging between 3 and 78 keV with ${\sim}1^{\prime }$ angular resolution. I review NuSTAR observations of the first three pulsar-wind nebulae (PWNe) to be examined in this way: the Crab Nebula, G21.5–0.9 and MSH 15–52. All three show spectral structure not previously known: spectral steepening in certain locations and overall source shrinkage with increasing photon energy. The Crab Nebula has different shrinkage rates along the torus and along the northwest counter-jet. The latter rate is similar to that for both the other sources (FWHM $\propto E^{m}$ with $m\sim -0.2$ ). I discuss implications of these results for models of particle transport in PWNe.}, number={05}, journal={JOURNAL OF PLASMA PHYSICS}, publisher={Cambridge University Press (CUP)}, author={Reynoldst, Stephen P.}, year={2016}, month={Oct} }
 @article{sankrit_raymond_blair_long_williams_borkowski_patnaude_reynolds_2016, title={SECOND EPOCH HUBBLE SPACE TELESCOPE OBSERVATIONS OF KEPLER'S SUPERNOVA REMNANT: THE PROPER MOTIONS OF BALMER FILAMENTS}, volume={817}, ISSN={["1538-4357"]}, url={https://doi.org/10.3847/0004-637X/817/1/36}, DOI={10.3847/0004-637x/817/1/36}, abstractNote={We report on the proper motions of Balmer-dominated filaments in Kepler’s supernova remnant using high resolution images obtained with the Hubble Space Telescope at two epochs separated by about 10 years. We use the improved proper motion measurements and revised values of shock velocities to derive a distance to Kepler of 5.1 − 0.7 + 0.8 ?> kpc. The main shock around the northern rim of the remnant has a typical speed of 1690 km s−1 and is encountering material with densities of about 8 cm−3. We find evidence for the variation of shock properties over small spatial scales, including differences in the driving pressures as the shock wraps around a curved cloud surface. We find that the Balmer filaments ahead of the ejecta knot on the northwest boundary of the remnant are becoming fainter and more diffuse. We also find that the Balmer filaments associated with circumstellar material in the interior regions of the remnant are due to shocks with significantly lower velocities and that the brightness variations among these filaments trace the density distribution of the material, which may have a disk-like geometry.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Sankrit, Ravi and Raymond, John C. and Blair, William P. and Long, Knox S. and Williams, Brian J. and Borkowski, Kazimierz J. and Patnaude, Daniel J. and Reynolds, Stephen P.}, year={2016}, month={Jan} }
 @article{tang_reynolds_ressler_2016, title={X-RAY AND GAMMA-RAY EMISSION FROM MIDDLE-AGED SUPERNOVA REMNANTS IN CAVITIES. I. SPHERICAL SYMMETRY}, volume={227}, ISSN={["1538-4365"]}, url={https://doi.org/10.3847/1538-4365/227/2/28}, DOI={10.3847/1538-4365/227/2/28}, abstractNote={We present analytical and numerical studies of models of supernova-remnant (SNR) blast waves in one spatial dimension expanding into uniform media and interacting with a denser cavity wall. We predict the nonthermal emission from such blast waves: synchrotron emission at radio and X-ray energies, and bremsstrahlung, inverse-Compton emission (from cosmic-microwave-background seed photons; ICCMB), and emission from the decay of mesons produced in inelastic collisions between accelerated ions and thermal gas, at GeV and TeV energies. Accelerated-particle spectra are assumed to be power laws with exponential cutoffs at energies limited by the remnant age or (for electrons, if lower) by radiative losses. We compare the results with those from homogeneous (“one-zone”) models. Such models give fair representations of the 1D results for uniform media, but cavity-wall interactions produce effects for which one-zone models are inadequate. We study the time evolution of SNR morphology and emission with time. Strong morphological differences exist between ICCMB and -decay emission; at some stages, the TeV emission can be dominated by the former and the GeV by the latter, resulting in strong energy dependence of morphology. Integrated gamma-ray spectra show apparent power laws of slopes that vary with time, but do not indicate the energy distribution of a single population of particles. As observational capabilities at GeV and TeV energies improve, spatial inhomogeneity in SNRs will need to be accounted for.}, number={2}, journal={ASTROPHYSICAL JOURNAL SUPPLEMENT SERIES}, publisher={American Astronomical Society}, author={Tang, Zhu and Reynolds, Stephen P. and Ressler, Sean M.}, year={2016}, month={Dec} }
 @article{lopez_grefenstette_reynolds_an_boggs_christensen_craig_eriksen_fryer_hailey_et al._2015, title={A SPATIALLY RESOLVED STUDY OF THE SYNCHROTRON EMISSION AND TITANIUM IN TYCHO'S SUPERNOVA REMNANT USING NuSTAR}, volume={814}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/814/2/132}, abstractNote={We report results from deep observations (∼750 ks) of Tycho's supernova remnant (SNR) with NuSTAR. Using these data, we produce narrow-band images over several energy bands to identify the regions producing the hardest X-rays and to search for radioactive decay line emission from 44Ti. We find that the hardest (>10 keV) X-rays are concentrated in the southwest of Tycho, where recent Chandra observations have revealed high emissivity “stripes” associated with particles accelerated to the knee of the cosmic-ray spectrum. We do not find evidence of 44Ti, and we set limits on its presence and distribution within the SNR. These limits correspond to an upper-limit 44Ti mass of M44 < 2.4 × 10−4 M⊙ for a distance of 2.3 kpc. We perform a spatially resolved spectroscopic analysis of 66 regions across Tycho. We map the best-fit rolloff frequency of the hard X-ray spectra, and we compare these results to measurements of the shock expansion and ambient density. We find that the highest energy electrons are accelerated at the lowest densities and in the fastest shocks, with a steep dependence of the rolloff frequency with shock velocity. Such a dependence is predicted by models where the maximum energy of accelerated electrons is limited by the age of the SNR rather than by synchrotron losses, but this scenario requires far lower magnetic field strengths than those derived from observations in Tycho. One way to reconcile these discrepant findings is through shock obliquity effects, and future observational work is necessary to explore the role of obliquity in the particle acceleration process.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, publisher={IOP Publishing}, author={Lopez, Laura A. and Grefenstette, Brian W. and Reynolds, Stephen P. and An, Hongjun and Boggs, Steven E. and Christensen, Finn E. and Craig, William W. and Eriksen, Kristoffer A. and Fryer, Chris L. and Hailey, Charles J. and et al.}, year={2015}, month={Dec} }
 @article{madsen_reynolds_harrison_an_boggs_christensen_craig_fryer_grefenstette_hailey_et al._2015, title={BROADBAND X-RAY IMAGING AND SPECTROSCOPY OF THE CRAB NEBULA AND PULSAR WITH NuSTAR}, volume={801}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/801/1/66}, abstractNote={We present broadband (3–78 keV) NuSTAR X-ray imaging and spectroscopy of the Crab nebula and pulsar. We show that while the phase-averaged and spatially integrated nebula + pulsar spectrum is a power law in this energy band, spatially resolved spectroscopy of the nebula finds a break at ∼9 keV in the spectral photon index of the torus structure with a steepening characterized by ΔΓ ∼ 0.25. We also confirm a previously reported steepening in the pulsed spectrum, and quantify it with a broken power law with break energy at ∼12 keV and ΔΓ ∼ 0.27. We present spectral maps of the inner 100″ of the remnant and measure the size of the nebula as a function of energy in seven bands. These results find that the rate of shrinkage with energy of the torus size can be fitted by a power law with an index of γ = 0.094 ± 0.018, consistent with the predictions of Kennel and Coroniti. The change in size is more rapid in the NW direction, coinciding with the counter-jet where we find the index to be a factor of two larger. NuSTAR observed the Crab during the latter part of a γ-ray flare, but found no increase in flux in the 3–78 keV energy band.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, publisher={IOP Publishing}, author={Madsen, Kristin K. and Reynolds, Stephen and Harrison, Fiona and An, Hongjun and Boggs, Steven and Christensen, Finn E. and Craig, William W. and Fryer, Chris L. and Grefenstette, Brian W. and Hailey, Charles J. and et al.}, year={2015}, month={Mar} }
 @article{tran_williams_petre_ressler_reynolds_2015, title={ENERGY DEPENDENCE OF SYNCHROTRON X-RAY RIMS IN TYCHO'S SUPERNOVA REMNANT}, volume={812}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/812/2/101}, abstractNote={Several young supernova remnants (SNRs) exhibit thin X-ray bright rims of synchrotron radiation at their forward shocks. Thin rims require strong magnetic field amplification beyond simple shock compression if rim widths are only limited by electron energy losses. But, magnetic field damping behind the shock could produce similarly thin rims with less extreme field amplification. Variation of rim width with energy may thus discriminate between competing influences on rim widths. We measured rim widths around Tycho's SNR in five energy bands using an archival 750 ks Chandra observation. Rims narrow with increasing energy and are well described by either loss-limited or damped scenarios, so X-ray rim width-energy dependence does not uniquely specify a model. But, radio counterparts to thin rims are not loss-limited and better reflect magnetic field structure. Joint radio and X-ray modeling favors magnetic damping in Tycho's SNR with damping lengths ∼1%–5% of remnant radius and magnetic field strengths ∼50–400 μG assuming Bohm diffusion. X-ray rim widths are ∼1% of remnant radius, somewhat smaller than inferred damping lengths. Electron energy losses are important in all models of X-ray rims, suggesting that the distinction between loss-limited and damped models is blurred in soft X-rays. All loss-limited and damping models require magnetic fields ≳20 μG, affirming the necessity of magnetic field amplification beyond simple compression.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, publisher={IOP Publishing}, author={Tran, Aaron and Williams, Brian J. and Petre, Robert and Ressler, Sean M. and Reynolds, Stephen P.}, year={2015}, month={Oct} }
 @article{grefenstette_reynolds_harrison_humensky_boggs_fryer_delaney_madsen_miyasaka_wik_et al._2015, title={LOCATING THE MOST ENERGETIC ELECTRONS IN CASSIOPEIA A}, volume={802}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/802/1/15}, abstractNote={We present deep (>2.4 Ms) observations of the Cassiopeia A supernova remnant with NuSTAR, which operates in the 3–79 keV bandpass and is the first instrument capable of spatially resolving the remnant above 15 keV. We find that the emission is not entirely dominated by the forward shock nor by a smooth “bright ring” at the reverse shock. Instead we find that the >15 keV emission is dominated by knots near the center of the remnant and dimmer filaments near the remnant’s outer rim. These regions are fit with unbroken power laws in the 15–50 keV bandpass, though the central knots have a steeper (Γ ∼ −3.35) spectrum than the outer filaments (Γ ∼ −3.06). We argue this difference implies that the central knots are located in the 3-D interior of the remnant rather than at the outer rim of the remnant and seen in the center due to projection effects. The morphology of >15 keV emission does not follow that of the radio emission nor that of the low energy (<12 keV) X-rays, leaving the origin of the >15 keV emission an open mystery. Even at the forward shock front we find less steepening of the spectrum than expected from an exponentially cut off electron distribution with a single cutoff energy. Finally, we find that the GeV emission is not associated with the bright features in the NuSTAR band while the TeV emission may be, suggesting that both hadronic and leptonic emission mechanisms may be at work.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, publisher={IOP Publishing}, author={Grefenstette, Brian W. and Reynolds, Stephen P. and Harrison, Fiona A. and Humensky, T. Brian and Boggs, Steven E. and Fryer, Chris L. and DeLaney, Tracey and Madsen, Kristin K. and Miyasaka, Hiromasa and Wik, Daniel R. and et al.}, year={2015}, month={Mar} }
 @article{zoglauer_reynolds_an_boggs_christensen_craig_fryer_grefenstette_harrison_hailey_et al._2015, title={THE HARD X-RAY VIEW OF THE YOUNG SUPERNOVA REMNANT G1.9+0.3}, volume={798}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/798/2/98}, abstractNote={NuSTAR observed G1.9+0.3, the youngest known supernova remnant in the Milky Way, for 350 ks and detected emission up to ∼30 keV. The remnant's X-ray morphology does not change significantly across the energy range from 3 to 20 keV. A combined fit between NuSTAR and Chandra shows that the spectrum steepens with energy. The spectral shape can be well fitted with synchrotron emission from a power-law electron energy distribution with an exponential cutoff with no additional features. It can also be described by a purely phenomenological model such as a broken power law or a power law with an exponential cutoff, though these descriptions lack physical motivation. Using a fixed radio flux at 1 GHz of 1.17 Jy for the synchrotron model, we get a column density of NH = (7.23 ± 0.07) × 1022 cm−2, a spectral index of α = 0.633 ± 0.003, and a roll-off frequency of νrolloff = (3.07 ± 0.18) × 1017 Hz. This can be explained by particle acceleration, to a maximum energy set by the finite remnant age, in a magnetic field of about 10 μG, for which our roll-off implies a maximum energy of about 100 TeV for both electrons and ions. Much higher magnetic-field strengths would produce an electron spectrum that was cut off by radiative losses, giving a much higher roll-off frequency that is independent of magnetic-field strength. In this case, ions could be accelerated to much higher energies. A search for 44Ti emission in the 67.9 keV line results in an upper limit of 1.5 × 10−5 photons cm−2 s−1 assuming a line width of 4.0 keV (1 sigma).}, number={2}, journal={ASTROPHYSICAL JOURNAL}, publisher={IOP Publishing}, author={Zoglauer, Andreas and Reynolds, Stephen P. and An, Hongjun and Boggs, Steven E. and Christensen, Finn E. and Craig, William W. and Fryer, Chris L. and Grefenstette, Brian W. and Harrison, Fiona A. and Hailey, Charles J. and et al.}, year={2015}, month={Jan} }
 @article{winkler_williams_reynolds_petre_long_katsuda_hwang_2014, title={A HIGH-RESOLUTION X-RAY AND OPTICAL STUDY OF SN 1006: ASYMMETRIC EXPANSION AND SMALL-SCALE STRUCTURE IN A TYPE IA SUPERNOVA REMNANT}, volume={781}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/781/2/65}, abstractNote={We introduce a deep (670 ks) X-ray survey of the entire SN 1006 remnant from the Chandra X-Ray Observatory, together with a deep Hα image of SN 1006 from the 4 m Blanco telescope at CTIO. Comparison with Chandra images from 2003 gives the first measurement of the X-ray proper motions around the entire periphery, carried out over a 9 yr baseline. We find that the expansion velocity varies significantly with azimuth. The highest velocity of ∼7400 km s−1 (almost 2.5 times that in the northwest (NW)) is found along the southeast (SE) periphery, where both the kinematics and the spectra indicate that most of the X-ray emission stems from ejecta that have been decelerated little, if at all. Asymmetries in the distribution of ejecta are seen on a variety of spatial scales. Si-rich ejecta are especially prominent in the SE quadrant, while O and Mg are more uniformly distributed, indicating large-scale asymmetries arising from the explosion itself. Neon emission is strongest in a sharp filament just behind the primary shock along the NW rim, where the pre-shock density is highest. Here the Ne is likely interstellar, while Ne within the shell may include a contribution from ejecta. Within the interior of the projected shell we find a few isolated “bullets” of what appear to be supernova ejecta that are immediately preceded by bowshocks seen in Hα, features that we interpret as ejecta knots that have reached relatively dense regions of the surrounding interstellar medium, but that appear in the interior in projection. Recent three-dimensional hydrodynamic models for Type Ia supernovae display small-scale features that strongly resemble the ones seen in X-rays in SN 1006; an origin in the explosion itself or from subsequent hydrodynamic instabilities both remain viable options. We have expanded the search for precursor X-ray emission ahead of a synchrotron-dominated shock front, as expected from diffusive shock acceleration theory, to numerous regions along both the northeast and southwest rims of the shell. Our data require that a precursor be thinner than about 3″, and fainter than about 5% of the post-shock peak. These limits suggest that the magnetic field is amplified by a factor of seven or more in a narrow precursor region, promoting diffusive particle acceleration.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Winkler, P. Frank and Williams, Brian J. and Reynolds, Stephen P. and Petre, Robert and Long, Knox S. and Katsuda, Satoru and Hwang, Una}, year={2014}, month={Feb} }
 @article{grefenstette_harrison_boggs_reynolds_fryer_madsen_wik_zoglauer_ellinger_alexander_et al._2014, title={Asymmetries in core-collapse supernovae from maps of radioactive Ti-44 in Cassiopeia A}, volume={506}, ISSN={["1476-4687"]}, DOI={10.1038/nature12997}, abstractNote={Asymmetry is required by most numerical simulations of stellar core-collapse explosions, but the form it takes differs significantly among models. The spatial distribution of radioactive (44)Ti, synthesized in an exploding star near the boundary between material falling back onto the collapsing core and that ejected into the surrounding medium, directly probes the explosion asymmetries. Cassiopeia A is a young, nearby, core-collapse remnant from which (44)Ti emission has previously been detected but not imaged. Asymmetries in the explosion have been indirectly inferred from a high ratio of observed (44)Ti emission to estimated (56)Ni emission, from optical light echoes, and from jet-like features seen in the X-ray and optical ejecta. Here we report spatial maps and spectral properties of the (44)Ti in Cassiopeia A. This may explain the unexpected lack of correlation between the (44)Ti and iron X-ray emission, the latter being visible only in shock-heated material. The observed spatial distribution rules out symmetric explosions even with a high level of convective mixing, as well as highly asymmetric bipolar explosions resulting from a fast-rotating progenitor. Instead, these observations provide strong evidence for the development of low-mode convective instabilities in core-collapse supernovae.}, number={7488}, journal={NATURE}, author={Grefenstette, B. W. and Harrison, F. A. and Boggs, S. E. and Reynolds, S. P. and Fryer, C. L. and Madsen, K. K. and Wik, D. R. and Zoglauer, A. and Ellinger, C. I. and Alexander, D. M. and et al.}, year={2014}, month={Feb}, pages={339-+} }
 @article{an_madsen_reynolds_kaspi_harrison_boggs_christensen_craig_fryer_grefenstette_et al._2014, title={HIGH-ENERGY X-RAY IMAGING OF THE PULSAR WIND NEBULA MSH 15-52: CONSTRAINTS ON PARTICLE ACCELERATION AND TRANSPORT}, volume={793}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/793/2/90}, abstractNote={We present the first images of the pulsar wind nebula (PWN) MSH 15−52 in the hard X-ray band (≳8 keV), as measured with the Nuclear Spectroscopic Telescope Array (NuSTAR). Overall, the morphology of the PWN as measured by NuSTAR in the 3–7 keV band is similar to that seen in Chandra high-resolution imaging. However, the spatial extent decreases with energy, which we attribute to synchrotron energy losses as the particles move away from the shock. The hard-band maps show a relative deficit of counts in the northern region toward the RCW 89 thermal remnant, with significant asymmetry. We find that the integrated PWN spectra measured with NuSTAR and Chandra suggest that there is a spectral break at 6 keV, which may be explained by a break in the synchrotron-emitting electron distribution at ∼200 TeV and/or imperfect cross calibration. We also measure spatially resolved spectra, showing that the spectrum of the PWN softens away from the central pulsar B1509−58, and that there exists a roughly sinusoidal variation of spectral hardness in the azimuthal direction. We discuss the results using particle flow models. We find non-monotonic structure in the variation with distance of spectral hardness within 50″ of the pulsar moving in the jet direction, which may imply particle and magnetic-field compression by magnetic hoop stress as previously suggested for this source. We also present two-dimensional maps of spectral parameters and find an interesting shell-like structure in the NH map. We discuss possible origins of the shell-like structure and their implications.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={An, Hongjun and Madsen, Kristin K. and Reynolds, Stephen P. and Kaspi, Victoria M. and Harrison, Fiona A. and Boggs, Steven E. and Christensen, Finn E. and Craig, William W. and Fryer, Chris L. and Grefenstette, Brian W. and et al.}, year={2014}, month={Oct} }
 @article{ressler_katsuda_reynolds_long_petre_williams_winkler_2014, title={MAGNETIC FIELD AMPLIFICATION IN THE THIN X-RAY RIMS OF SN 1006}, volume={790}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/790/2/85}, abstractNote={Several young supernova remnants, including SN 1006, emit synchrotron X-rays in narrow filaments, hereafter thin rims, along their periphery. The widths of these rims imply 50–100 μG fields in the region immediately behind the shock, far larger than expected for the interstellar medium compressed by unmodified shocks, assuming electron radiative losses limit rim widths. However, magnetic field damping could also produce thin rims. Here we review the literature on rim width calculations, summarizing the case for magnetic field amplification. We extend these calculations to include an arbitrary power-law dependence of the diffusion coefficient on energy, D∝Eμ. Loss-limited rim widths should shrink with increasing photon energy, while magnetic-damping models predict widths almost independent of photon energy. We use these results to analyze Chandra observations of SN 1006, in particular the southwest limb. We parameterize the FWHM in terms of energy as FWHM . Filament widths in SN 1006 decrease with energy; mE ∼ −0.3 to −0.8, implying magnetic field amplification by factors of 10–50, above the factor of four expected in strong unmodified shocks. For SN 1006, the rapid shrinkage rules out magnetic damping models. It also favors short mean free paths (small diffusion coefficients) and strong dependence of D on energy (μ ⩾ 1).}, number={2}, journal={ASTROPHYSICAL JOURNAL}, publisher={IOP Publishing}, author={Ressler, Sean M. and Katsuda, Satoru and Reynolds, Stephen P. and Long, Knox S. and Petre, Robert and Williams, Brian J. and Winkler, P. Frank}, year={2014}, month={Aug} }
 @article{carlton_borkowski_reynolds_hwang_petre_green_krishnamurthy_willett_2014, title={NONUNIFORM EXPANSION OF THE YOUNGEST GALACTIC SUPERNOVA REMNANT G1.9+0.3}, volume={790}, ISSN={["2041-8213"]}, url={https://doi.org/10.1088/2041-8205/790/2/L18}, DOI={10.1088/2041-8205/790/2/l18}, abstractNote={We report measurements of X-ray expansion of the youngest Galactic supernova remnant, G1.9+0.3, using Chandra observations in 2007, 2009, and 2011. The measured rates strongly deviate from uniform expansion, decreasing radially by about 60% along the X-ray bright SE-NW axis from 0.84% +/- 0.06% per yr to 0.52% +/- 0.03% per yr. This corresponds to undecelerated ages of 120-190 yr, confirming the young age of G1.9+0.3, and implying a significant deceleration of the blast wave. The synchrotron-dominated X-ray emission brightens at a rate of 1.9% +/- 0.4% per yr. We identify bright outer and inner rims with the blast wave and reverse shock, respectively. Sharp density gradients in either ejecta or ambient medium are required to produce the sudden deceleration of the reverse shock or the blast wave implied by the large spread in expansion ages. The blast wave could have been decelerated recently by an encounter with a modest density discontinuity in the ambient medium, such as found at a wind termination shock, requiring strong mass loss in the progenitor. Alternatively, the reverse shock might have encountered an order-of-magnitude density discontinuity within the ejecta, such as found in pulsating delayed-detonation Type Ia models. We demonstrate that the blast wave is much more decelerated than the reverse shock in these models for remnants at ages similar to G1.9+0.3. Similar effects may also be produced by dense shells possibly associated with high-velocity features in Type Ia spectra. Accounting for the asymmetry of G1.9+0.3 will require more realistic 3D Type Ia models.}, number={2}, journal={ASTROPHYSICAL JOURNAL LETTERS}, publisher={IOP Publishing}, author={Carlton, A. K. and Borkowski, K. J. and Reynolds, Stephen and Hwang, U. and Petre, R. and Green, D. A. and Krishnamurthy, K. and Willett, R.}, year={2014}, month={Aug} }
 @article{nynka_hailey_reynolds_an_baganoff_boggs_christensen_craig_gotthelf_grefenstette_et al._2014, title={NuSTAR STUDY OF HARD X-RAY MORPHOLOGY AND SPECTROSCOPY OF PWN G21.5–0.9}, volume={789}, DOI={10.1088/0004-637X/789/1/72}, abstractNote={We present NuSTAR high-energy X-ray observations of the pulsar wind nebula (PWN)/supernova remnant G21.5−0.9. We detect integrated emission from the nebula up to ∼40 keV, and resolve individual spatial features over a broad X-ray band for the first time. The morphology seen by NuSTAR agrees well with that seen by XMM-Newton and Chandra below 10 keV. At high energies, NuSTAR clearly detects non-thermal emission up to ∼20 keV that extends along the eastern and northern rim of the supernova shell. The broadband images clearly demonstrate that X-ray emission from the North Spur and Eastern Limb results predominantly from non-thermal processes. We detect a break in the spatially integrated X-ray spectrum at ∼9 keV that cannot be reproduced by current spectral energy distribution models, implying either a more complex electron injection spectrum or an additional process such as diffusion compared to what has been considered in previous work. We use spatially resolved maps to derive an energy-dependent cooling length scale, L(E)∝Em with m = −0.21 ± 0.01. We find this to be inconsistent with the model for the morphological evolution with energy described by Kennel & Coroniti. This value, along with the observed steepening in power-law index between radio and X-ray, can be quantitatively explained as an energy-loss spectral break in the simple scaling model of Reynolds, assuming particle advection dominates over diffusion. This interpretation requires a substantial departure from spherical magnetohydrodynamic, magnetic-flux-conserving outflow, most plausibly in the form of turbulent magnetic-field amplification.}, number={1}, journal={The Astrophysical Journal}, publisher={IOP Publishing}, author={Nynka, Melania and Hailey, Charles J. and Reynolds, Stephen P. and An, Hongjun and Baganoff, Frederick K. and Boggs, Steven E. and Christensen, Finn E. and Craig, William W. and Gotthelf, Eric V. and Grefenstette, Brian W. and et al.}, year={2014}, month={Jun}, pages={72} }
 @article{ofek_zoglauer_boggs_barriere_reynolds_fryer_harrison_cenko_kulkarni_gal-yam_et al._2014, title={SN 2010jl: OPTICAL TO HARD X-RAY OBSERVATIONS REVEAL AN EXPLOSION EMBEDDED IN A TEN SOLAR MASS COCOON}, volume={781}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/781/1/42}, abstractNote={Some supernovae (SNe) may be powered by the interaction of the SN ejecta with a large amount of circumstellar matter (CSM). However, quantitative estimates of the CSM mass around such SNe are missing when the CSM material is optically thick. Specifically, current estimators are sensitive to uncertainties regarding the CSM density profile and the ejecta velocity. Here we outline a method to measure the mass of the optically thick CSM around such SNe. We present new visible-light and X-ray observations of SN 2010jl (PTF 10aaxf), including the first detection of an SN in the hard X-ray band using NuSTAR. The total radiated luminosity of SN 2010jl is extreme—at least 9 × 1050 erg. By modeling the visible-light data, we robustly show that the mass of the circumstellar material within ∼1016 cm of the progenitor of SN 2010jl was in excess of 10 M☉. This mass was likely ejected tens of years prior to the SN explosion. Our modeling suggests that the shock velocity during shock breakout was ∼6000 km s−1, decelerating to ∼2600 km s−1 about 2 yr after maximum light. Furthermore, our late-time NuSTAR and XMM spectra of the SN presumably provide the first direct measurement of SN shock velocity 2 yr after the SN maximum light—measured to be in the range of 2000–4500 km s−1 if the ions and electrons are in equilibrium, and ≳ 2000 km s−1 if they are not in equilibrium. This measurement is in agreement with the shock velocity predicted by our modeling of the visible-light data. Our observations also show that the average radial density distribution of the CSM roughly follows an r−2 law. A possible explanation for the ≳ 10 M☉ of CSM and the wind-like profile is that they are the result of multiple pulsational pair instability events prior to the SN explosion, separated from each other by years.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Ofek, Eran O. and Zoglauer, Andreas and Boggs, Steven E. and Barriere, Nicolas M. and Reynolds, Stephen P. and Fryer, Chris L. and Harrison, Fiona A. and Cenko, S. Bradley and Kulkarni, Shrinivas R. and Gal-Yam, Avishay and et al.}, year={2014}, month={Jan} }
 @article{williams_borkowski_reynolds_ghavamian_raymond_long_blair_sankrit_winkler_hendrick_et al._2014, title={SPITZER OBSERVATIONS OF THE TYPE IA SUPERNOVA REMNANT N103B: KEPLER'S OLDER COUSIN?}, volume={790}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/790/2/139}, abstractNote={We report results from Spitzer observations of SNR 0509-68.7, also known as N103B, a young Type Ia supernova remnant (SNR) in the Large Magellanic Cloud (LMC) that shows interaction with a dense medium in its western hemisphere. Our images show that N103B has strong IR emission from warm dust in the post-shock environment. The post-shock gas density we derive, 45 cm−3, is much higher than in other Type Ia remnants in the LMC, though a lack of spatial resolution may bias measurements toward regions of higher than average density. This density is similar to that in Kepler's SNR, a Type Ia interacting with a circumstellar medium (CSM). Optical images show Hα emission along the entire periphery of the western portion of the shock, with [O iii] and [S ii] lines emitted from a few dense clumps of material where the shock has become radiative. The dust is silicate in nature, though standard silicate dust models fail to reproduce the “18 μm” silicate feature that peaks instead at 17.3 μm. We propose that the dense material is circumstellar material lost from the progenitor system, as with Kepler. If the CSM interpretation is correct, this remnant would become the second member, along with Kepler, of a class of Type Ia remnants characterized by interaction with a dense CSM hundreds of years post-explosion. A lack of N enhancement eliminates symbiotic asymptotic giant branch progenitors. The white dwarf companion must have been relatively unevolved at the time of the explosion.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, publisher={IOP Publishing}, author={Williams, Brian J. and Borkowski, Kazimierz J. and Reynolds, Stephen P. and Ghavamian, Parviz and Raymond, John C. and Long, Knox S. and Blair, William P. and Sankrit, Ravi and Winkler, P. Frank and Hendrick, Sean P. and et al.}, year={2014}, month={Aug} }
 @article{williams_borkowski_ghavamian_hewitt_mao_petre_reynolds_blondin_2013, title={AZIMUTHAL DENSITY VARIATIONS AROUND THE RIM OF TYCHO's SUPERNOVA REMNANT}, volume={770}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/770/2/129}, abstractNote={Spitzer images of Tycho's supernova remnant in the mid-infrared reveal limb-brightened emission from the entire periphery of the shell and faint filamentary structures in the interior. As with other young remnants, this emission is produced by dust grains, warmed to ∼100 K in the post-shock environment by collisions with energetic electrons and ions. The ratio of the 70 to 24 μm fluxes is a diagnostic of the dust temperature, which in turn is a sensitive function of the plasma density. We find significant variations in the 70/24 flux ratio around the periphery of Tycho's forward shock, implying order-of-magnitude variations in density. While some of these are likely localized interactions with dense clumps of the interstellar medium (ISM), we find an overall gradient in the ambient density surrounding Tycho, with densities 3–10 times higher in the northeast than in the southwest. This large density gradient is qualitatively consistent with the variations in the proper motion of the shock observed in radio and X-ray studies. Overall, the mean ISM density around Tycho is quite low (∼0.1–0.2 cm−3), consistent with the lack of thermal X-ray emission observed at the forward shock. We perform two-dimensional hydrodynamic simulations of a Type Ia supernova expanding into a density gradient in the ISM, and find that the overall round shape of the remnant is still easily achievable, even for explosions into significant gradients. However, this leads to an offset of the center of the explosion from the geometric center of the remnant of up to 20%, although lower values of 10% are preferred. The best match with hydrodynamical simulations is achieved if Tycho is located at a large (3–4 kpc) distance in a medium with a mean preshock density of ∼0.2 cm−3. Such preshock densities are obtained for highly (≳ 50%) porous ISM grains.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Williams, Brian J. and Borkowski, Kazimierz J. and Ghavamian, Parviz and Hewitt, John W. and Mao, S. Alwin and Petre, Robert and Reynolds, Stephen P. and Blondin, John M.}, year={2013}, month={Jun} }
 @article{borkowski_reynolds_hwang_green_petre_krishnamurthy_willett_2013, title={SUPERNOVA EJECTA IN THE YOUNGEST GALACTIC SUPERNOVA REMNANT G1.9+0.3}, volume={771}, ISSN={["2041-8213"]}, DOI={10.1088/2041-8205/771/1/l9}, abstractNote={G1.9+0.3 is the youngest known Galactic supernova remnant (SNR), with an estimated supernova (SN) explosion date of ∼1900, and most likely located near the Galactic center. Only the outermost ejecta layers with free-expansion velocities ≳18,000 km s−1 have been shocked so far in this dynamically young, likely Type Ia SNR. A long (980 ks) Chandra observation in 2011 allowed spatially resolved spectroscopy of heavy-element ejecta. We denoised Chandra data with the spatio-spectral method of Krishnamurthy et al., and used a wavelet-based technique to spatially localize thermal emission produced by intermediate-mass elements (IMEs; Si and S) and iron. The spatial distribution of both IMEs and Fe is extremely asymmetric, with the strongest ejecta emission in the northern rim. Fe Kα emission is particularly prominent there, and fits with thermal models indicate strongly oversolar Fe abundances. In a localized, outlying region in the northern rim, IMEs are less abundant than Fe, indicating that undiluted Fe-group elements (including 56Ni) with velocities >18,000 km s−1 were ejected by this SN. However, in the inner west rim, we find Si- and S-rich ejecta without any traces of Fe, so high-velocity products of O-burning were also ejected. G1.9+0.3 appears similar to energetic Type Ia SNe such as SN 2010jn where iron-group elements at such high free-expansion velocities have been recently detected. The pronounced asymmetry in the ejecta distribution and abundance inhomogeneities are best explained by a strongly asymmetric SN explosion, similar to those produced in some recent three-dimensional delayed-detonation Type Ia models.}, number={1}, journal={ASTROPHYSICAL JOURNAL LETTERS}, author={Borkowski, Kazimierz J. and Reynolds, Stephen P. and Hwang, Una and Green, David A. and Petre, Robert and Krishnamurthy, Kalyani and Willett, Rebecca}, year={2013}, month={Jul} }
 @article{winkler_williams_blair_borkowski_ghavamian_long_raymond_reynolds_2013, title={THE FIRST REPORTED INFRARED EMISSION FROM THE SN 1006 REMNANT}, volume={764}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1088/0004-637X/764/2/156}, DOI={10.1088/0004-637X/764/2/156}, abstractNote={We report results of infrared imaging and spectroscopic observations of the SN 1006 remnant, carried out with the Spitzer Space Telescope. The 24 μm image from Multiband Imaging Photometer for Spitzer clearly shows faint filamentary emission along the northwest rim of the remnant shell, nearly coincident with the Balmer filaments that delineate the present position of the expanding shock. The 24 μm emission traces the Balmer filaments almost perfectly but lies a few arcsec within, indicating an origin in interstellar dust heated by the shock. Subsequent decline in the IR behind the shock is presumably due largely to grain destruction through sputtering. The emission drops far more rapidly than current models predict, however, even for a higher proportion of small grains than would be found closer to the Galactic plane. The rapid drop may result in part from a grain density that has always been lower—a relic effect from an earlier epoch when the shock was encountering a lower density—but higher grain destruction rates still seem to be required. Spectra from three positions along the NW filament from the Infrared Spectrometer instrument all show only a featureless continuum, consistent with thermal emission from warm dust. The dust-to-gas mass ratio in the pre-shock interstellar medium (ISM) is lower than that expected for the Galactic ISM—as has also been observed in the analysis of IR emission from other supernova remnants, but whose cause remains unclear. As with other Type Ia supernova (SN Ia) remnants, SN 1006 shows no evidence for dust grain formation in the SN ejecta.}, number={2}, journal={The Astrophysical Journal}, publisher={IOP Publishing}, author={Winkler, P. Frank and Williams, Brian J. and Blair, William P. and Borkowski, Kazimierz J. and Ghavamian, Parviz and Long, Knox S. and Raymond, John C. and Reynolds, Stephen P.}, year={2013}, month={Feb}, pages={156} }
 @article{harrison_craig_christensen_hailey_zhang_boggs_stern_cook_forster_giommi_et al._2013, title={The Nuclear Spectroscopic Telescope Array (NuSTAR) High-Energy X-ray Mission}, volume={770}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1088/0004-637X/770/2/103}, DOI={10.1088/0004-637X/770/2/103}, abstractNote={The Nuclear Spectroscopic Telescope Array (NuSTAR) mission, launched on 2012 June 13, is the first focusing high-energy X-ray telescope in orbit. NuSTAR operates in the band from 3 to 79 keV, extending the sensitivity of focusing far beyond the ∼10 keV high-energy cutoff achieved by all previous X-ray satellites. The inherently low background associated with concentrating the X-ray light enables NuSTAR to probe the hard X-ray sky with a more than 100-fold improvement in sensitivity over the collimated or coded mask instruments that have operated in this bandpass. Using its unprecedented combination of sensitivity and spatial and spectral resolution, NuSTAR will pursue five primary scientific objectives: (1) probe obscured active galactic nucleus (AGN) activity out to the peak epoch of galaxy assembly in the universe (at z ≲ 2) by surveying selected regions of the sky; (2) study the population of hard X-ray-emitting compact objects in the Galaxy by mapping the central regions of the Milky Way; (3) study the non-thermal radiation in young supernova remnants, both the hard X-ray continuum and the emission from the radioactive element 44Ti; (4) observe blazars contemporaneously with ground-based radio, optical, and TeV telescopes, as well as with Fermi and Swift, to constrain the structure of AGN jets; and (5) observe line and continuum emission from core-collapse supernovae in the Local Group, and from nearby Type Ia events, to constrain explosion models. During its baseline two-year mission, NuSTAR will also undertake a broad program of targeted observations. The observatory consists of two co-aligned grazing-incidence X-ray telescopes pointed at celestial targets by a three-axis stabilized spacecraft. Deployed into a 600 km, near-circular, 6° inclination orbit, the observatory has now completed commissioning, and is performing consistent with pre-launch expectations. NuSTAR is now executing its primary science mission, and with an expected orbit lifetime of 10 yr, we anticipate proposing a guest investigator program, to begin in late 2014.}, number={2}, journal={The Astrophysical Journal}, publisher={IOP Publishing}, author={Harrison, Fiona A. and Craig, William W. and Christensen, Finn E. and Hailey, Charles J. and Zhang, William W. and Boggs, Steven E. and Stern, Daniel and Cook, W. Rick and Forster, Karl and Giommi, Paolo and et al.}, year={2013}, month={May}, pages={103} }
 @article{burkey_reynolds_borkowski_blondin_2013, title={X-RAY EMISSION FROM STRONGLY ASYMMETRIC CIRCUMSTELLAR MATERIAL IN THE REMNANT OF KEPLER'S SUPERNOVA}, volume={764}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1088/0004-637X/764/1/63}, DOI={10.1088/0004-637x/764/1/63}, abstractNote={Kepler's supernova remnant resulted from a thermonuclear explosion, but is interacting with circumstellar material (CSM) lost from the progenitor system. We describe a statistical technique for isolating X-ray emission due to CSM from that due to shocked ejecta. Shocked CSM coincides well in position with 24 μm emission seen by Spitzer. We find most CSM to be distributed along the bright north rim, but substantial concentrations are also found projected against the center of the remnant, roughly along a diameter with position angle ∼100°. We interpret this as evidence for a disk distribution of CSM before the supernova, with the line of sight to the observer roughly in the disk plane. We present two-dimensional hydrodynamic simulations of this scenario in qualitative agreement with the observed CSM morphology. Our observations require Kepler to have originated in a close binary system with an asymptotic giant branch star companion.}, number={1}, journal={The Astrophysical Journal}, publisher={IOP Publishing}, author={Burkey, Mary T. and Reynolds, Stephen P. and Borkowski, Kazimierz J. and Blondin, John M.}, year={2013}, month={Jan}, pages={63} }
 @article{katsuda_long_petre_reynolds_williams_winkler_2013, title={X-RAY PROPER MOTIONS AND SHOCK SPEEDS ALONG THE NORTHWEST RIM OF SN 1006}, volume={763}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/763/2/85}, abstractNote={We report the results of an X-ray proper-motion measurement for the NW rim of SN 1006, carried out by comparing Chandra observations from 2001 to 2012. The NW limb has predominantly thermal X-ray emission, and it is the only location in SN 1006 with significant optical emission: a thin, Balmer-dominated filament. For most of the NW rim, the proper motion is ≈0.″30 yr−1, essentially the same as has been measured from the Hα filament. Isolated regions of the NW limb are dominated by nonthermal emission, and here the proper motion is much higher, 0.″49 yr−1, close to the value measured in X-rays along the much brighter NE limb, where the X-rays are overwhelmingly nonthermal. At the 2.2 kpc distance to SN 1006, the proper motions imply shock velocities of ∼3000 km s−1 and ∼5000 km s−1 in the thermal and nonthermal regions, respectively. A lower velocity behind the Hα filament is consistent with the picture that SN 1006 is encountering denser gas in the NW, as is also suggested by its overall morphology. In the thermally dominated portion of the X-ray shell, we also see an offset in the radial profiles at different energies; the 0.5–0.6 keV peak dominated by O vii is closer to the shock front than that of the 0.8–3 keV emission—due to the longer times for heavier elements to reach ionization states where they produce strong X-ray emission.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Katsuda, Satoru and Long, Knox S. and Petre, Robert and Reynolds, Stephen P. and Williams, Brian J. and Winkler, P. Frank}, year={2013}, month={Feb} }
 @article{williams_borkowski_reynolds_ghavamian_blair_long_sankrit_2012, title={DUST IN A TYPE Ia SUPERNOVA PROGENITOR: SPITZER SPECTROSCOPY OF KEPLER'S SUPERNOVA REMNANT}, volume={755}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/755/1/3}, abstractNote={Characterization of the relatively poorly understood progenitor systems of Type Ia supernovae is of great importance in astrophysics, particularly given the important cosmological role that these supernovae play. Kepler's supernova remnant, the result of a Type Ia supernova, shows evidence for an interaction with a dense circumstellar medium (CSM), suggesting a single-degenerate progenitor system. We present 7.5–38 μm infrared (IR) spectra of the remnant, obtained with the Spitzer Space Telescope, dominated by emission from warm dust. Broad spectral features at 10 and 18 μm, consistent with various silicate particles, are seen throughout. These silicates were likely formed in the stellar outflow from the progenitor system during the asymptotic giant branch stage of evolution, and imply an oxygen-rich chemistry. In addition to silicate dust, a second component, possibly carbonaceous dust, is necessary to account for the short-wavelength Infrared Spectrograph and Infrared Array Camera data. This could imply a mixed chemistry in the atmosphere of the progenitor system. However, non-spherical metallic iron inclusions within silicate grains provide an alternative solution. Models of collisionally heated dust emission from fast shocks (>1000 km s−1) propagating into the CSM can reproduce the majority of the emission associated with non-radiative filaments, where dust temperatures are ∼80–100 K, but fail to account for the highest temperatures detected, in excess of 150 K. We find that slower shocks (a few hundred km s−1) into moderate density material (n0 ∼ 50–250 cm−3) are the only viable source of heating for this hottest dust. We confirm the finding of an overall density gradient, with densities in the north being an order of magnitude greater than those in the south.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Williams, Brian J. and Borkowski, Kazimierz J. and Reynolds, Stephen P. and Ghavamian, Parviz and Blair, William P. and Long, Knox S. and Sankrit, Ravi}, year={2012}, month={Aug} }
 @article{zajczyk_gallant_slane_reynolds_bandiera_gouiffes_le floc'h_comeron_miramond_2012, title={Infrared imaging and polarimetric observations of the pulsar wind nebula in SNR G21.5-0.9}, volume={542}, ISSN={["0004-6361"]}, DOI={10.1051/0004-6361/201117194}, abstractNote={We present infrared observations of the supernova remnant G21.5-0.9 with the Very Large Telescope, the Canada-France-Hawaii Telescope and the Spitzer Space Telescope. Using the VLT/ISAAC camera equipped with a narrow-band [Fe II] 1.64 μm filter the entire pulsar wind nebula in SNR G21.5-0.9 was imaged. This led to detection of iron line-emitting material in the shape of a broken ring-like structure following the nebula’s edge. The detected emission is limb-brightened. We also detect the compact nebula}, journal={ASTRONOMY & ASTROPHYSICS}, author={Zajczyk, A. and Gallant, Y. A. and Slane, P. and Reynolds, S. P. and Bandiera, R. and Gouiffes, C. and Le Floc'h, E. and Comeron, F. and Miramond, L. Koch}, year={2012}, month={Jun} }
 @misc{reynolds_gaensler_bocchino_2012, title={Magnetic Fields in Supernova Remnants and Pulsar-Wind Nebulae}, volume={166}, ISSN={["1572-9672"]}, DOI={10.1007/s11214-011-9775-y}, abstractNote={We review the observations of supernova remnants (SNRs) and pulsar-wind nebulae (PWNe) that give information on the strength and orientation of magnetic fields. Radio polarimetry gives the degree of order of magnetic fields, and the orientation of the ordered component. Many young shell supernova remnants show evidence for synchrotron X-ray emission. The spatial analysis of this emission suggests that magnetic fields are amplified by one to two orders of magnitude in strong shocks. Detection of several remnants in TeV gamma rays implies a lower limit on the magnetic-field strength (or a measurement, if the emission process is inverse-Compton upscattering of cosmic microwave background photons). Upper limits to GeV emission similarly provide lower limits on magnetic-field strengths. In the historical shell remnants, lower limits on B range from 25 to 1000 microGauss. Two remnants show variability of synchrotron X-ray emission with a timescale of years. If this timescale is the electron-acceleration or radiative loss timescale, magnetic fields of order 1 mG are also implied. In pulsar-wind nebulae, equipartition arguments and dynamical modeling can be used to infer magnetic-field strengths anywhere from about 5 microGauss to 1 mG. Polarized fractions are considerably higher than in SNRs, ranging to 50 or 60% in some cases; magnetic-field geometries often suggest a toroidal structure around the pulsar, but this is not universal. Viewing-angle effects undoubtedly play a role. MHD models of radio emission in shell SNRs show that different orientations of upstream magnetic field, and different assumptions about electron acceleration, predict different radio morphology. In the remnant of SN 1006, such comparisons imply a magnetic-field orientation connecting the bright limbs, with a non-negligible gradient of its strength across the remnant.}, number={1-4}, journal={SPACE SCIENCE REVIEWS}, author={Reynolds, Stephen P. and Gaensler, B. M. and Bocchino, Fabrizio}, year={2012}, month={May}, pages={231–261} }
 @article{williams_borkowski_reynolds_ghavamian_raymond_long_blair_sankrit_smith_points_et al._2011, title={DUSTY BLAST WAVES OF TWO YOUNG LARGE MAGELLANIC CLOUD SUPERNOVA REMNANTS: CONSTRAINTS ON POST-SHOCK COMPRESSION}, volume={729}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/729/1/65}, abstractNote={We present results from mid-IR spectroscopic observations of two young supernova remnants (SNRs) in the Large Magellanic Cloud made with the Spitzer Space Telescope. We imaged SNRs B0509-67.5 and B0519-69.0 with Spitzer in 2005, and follow-up spectroscopy presented here confirms the presence of warm, shock-heated dust, with no lines present in the spectrum. We use model fits to Spitzer Infrared Spectrograph (IRS) data to estimate the density of the post-shock gas. Both remnants show asymmetries in the infrared images, and we interpret bright spots as places where the forward shock is running into material that is several times denser than elsewhere. The densities we infer for these objects depend on the grain composition assumed, and we explore the effects of differing grain porosity on the model fits. We also analyze archival XMM-Newton RGS spectroscopic data, where both SNRs show strong lines of both Fe and Si, coming from ejecta, as well as strong O lines, which may come from ejecta or shocked ambient medium. We use model fits to IRS spectra to predict X-ray O line strengths for various grain models and values of the shock compression ratio. For 0509-67.5, we find that compact (solid) grain models require nearly all O lines in X-ray spectra to originate in reverse-shocked ejecta. Porous dust grains would lower the strength of ejecta lines relative to those arising in the shocked ambient medium. In 0519-69.0, we find significant evidence for a higher than standard compression ratio of 12, implying efficient cosmic-ray acceleration by the blast wave. A compact grain model is favored over porous grain models. We find that the dust-to-gas mass ratio of the ambient medium is significantly lower than what is expected in the interstellar medium.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Williams, Brian J. and Borkowski, Kazimierz J. and Reynolds, Stephen P. and Ghavamian, Parviz and Raymond, John C. and Long, Knox S. and Blair, William P. and Sankrit, Ravi and Smith, R. Chris and Points, Sean and et al.}, year={2011}, month={Mar} }
 @article{chevalier_reynolds_2011, title={PULSAR WIND NEBULAE WITH THICK TOROIDAL STRUCTURE}, volume={740}, ISSN={["2041-8205"]}, DOI={10.1088/2041-8205/740/1/l26}, abstractNote={We investigate a class of pulsar wind nebulae that show synchrotron emission from a thick toroidal structure. The best studied such object is the small radio and X-ray nebula around the Vela pulsar, which can be interpreted as the result of interaction of a mildly supersonic inward flow with the recent pulsar wind. Such a flow near the center of a supernova remnant can be produced in a transient phase when the reverse shock reaches the center of the remnant. Other nebulae with a thick toroidal structure are G106.6+2.9 and G76.9+1.0. Their structure contrasts with young pulsar nebulae like the Crab Nebula and 3C 38, which show a more chaotic, filamentary structure in the synchrotron emission. In both situations, a torus-jet structure is present where the pulsar wind passes through a termination shock, indicating the flow is initially toroidal. We suggest that the difference is due to the Rayleigh–Taylor instability that operates when the outer boundary of the nebula is accelerating into freely expanding supernova ejecta. The instability gives rise to mixing in the Crab and related objects, but is not present in the nebulae with thick toroidal regions.}, number={1}, journal={ASTROPHYSICAL JOURNAL LETTERS}, author={Chevalier, Roger A. and Reynolds, Stephen P.}, year={2011}, month={Oct} }
 @article{reynolds_2011, title={Particle acceleration in supernova-remnant shocks}, volume={336}, ISSN={["0004-640X"]}, DOI={10.1007/s10509-010-0559-8}, abstractNote={It has been known for over 50 years that the radio emission from shell supernova remnants (SNRs) indicates the presence of electrons with energies in the GeV range emitting synchrotron radiation. The discovery of nonthermal X-ray emission from supernova remnants is now 30 years old, and its interpretation as the extension of the radio synchrotron spectrum requires electrons with energies of up to 100 TeV. SNRs are now detected at GeV and TeV photon energies as well. Strong suggestions of the presence of energetic ions exist, but conclusive evidence remains elusive. Several arguments suggest that magnetic fields in SNRs are amplified by orders of magnitude from their values in the ambient interstellar medium. Supernova remnants are thus an excellent laboratory in which to study processes taking place in very high Mach-number shocks. I review the observations of high-energy emission from SNRs, and the theoretical framework in which those observations are interpreted.}, number={1}, journal={ASTROPHYSICS AND SPACE SCIENCE}, author={Reynolds, S. P.}, year={2011}, month={Nov}, pages={257–262} }
 @article{williams_blair_blondin_borkowski_ghavamian_long_raymond_reynolds_rho_winkler_et al._2011, title={RCW 86: A TYPE Ia SUPERNOVA IN A WIND-BLOWN BUBBLE}, volume={741}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/741/2/96}, abstractNote={We report results from a multi-wavelength analysis of the Galactic supernova remnant RCW 86, the proposed remnant of the supernova of 185 A.D. We show new infrared observations from the Spitzer Space Telescope and the Wide-Field Infrared Survey Explorer, where the entire shell is detected at 24 and 22 μm. We fit the infrared flux ratios with models of collisionally heated ambient dust, finding post-shock gas densities in the non-radiative shocks of 2.4 and 2.0 cm−3 in the southwest (SW) and northwest (NW) portions of the remnant, respectively. The Balmer-dominated shocks around the periphery of the shell, large amount of iron in the X-ray-emitting ejecta, and lack of a compact remnant support a Type Ia origin for this remnant. From hydrodynamic simulations, the observed characteristics of RCW 86 are successfully reproduced by an off-center explosion in a low-density cavity carved by the progenitor system. This would make RCW 86 the first known case of a Type Ia supernova in a wind-blown bubble. The fast shocks (>3000 km s−1) observed in the northeast are propagating in the low-density bubble, where the shock is just beginning to encounter the shell, while the slower shocks elsewhere have already encountered the bubble wall. The diffuse nature of the synchrotron emission in the SW and NW is due to electrons that were accelerated early in the lifetime of the remnant, when the shock was still in the bubble. Electrons in a bubble could produce gamma rays by inverse-Compton scattering. The wind-blown bubble scenario requires a single-degenerate progenitor, which should leave behind a companion star.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Williams, B. J. and Blair, W. P. and Blondin, J. M. and Borkowski, K. J. and Ghavamian, P. and Long, K. S. and Raymond, J. C. and Reynolds, S. P. and Rho, J. and Winkler, P. F. and et al.}, year={2011}, month={Nov} }
 @article{temim_slane_reynolds_raymond_borkowski_2010, title={DEEP CHANDRA OBSERVATIONS OF THE CRAB-LIKE PULSAR WIND NEBULA G54.1+0.3 AND SPITZER SPECTROSCOPY OF THE ASSOCIATED INFRARED SHELL}, volume={710}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637X/710/1/309}, abstractNote={G54.1+0.3 is a young pulsar wind nebula (PWN), closely resembling the Crab, for which no thermal shell emission has been detected in X-rays. Recent Spitzer observations revealed an infrared (IR) shell containing a dozen point sources arranged in a ring-like structure, previously proposed to be young stellar objects. An extended knot of emission located in the NW part of the shell appears to be aligned with the pulsar's X-ray jet, suggesting a possible interaction with the shell material. Surprisingly, the IR spectrum of the knot resembles the spectrum of freshly formed dust in Cas A, and is dominated by an unidentified dust emission feature at 21 μm. The spectra of the shell also contain various emission lines and show that some are significantly broadened, suggesting that they originate in rapidly expanding supernova (SN) ejecta. We present the first evidence that the PWN is driving shocks into expanding SN ejecta and we propose an alternative explanation for the origin of the IR emission in which the shell is composed entirely of SN ejecta. In this scenario, the freshly formed SN dust is being heated by early-type stars belonging to a cluster in which the SN exploded. Simple dust models show that this interpretation can give rise to the observed shell emission and the IR point sources.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Temim, Tea and Slane, Patrick and Reynolds, Stephen P. and Raymond, John C. and Borkowski, Kazimierz J.}, year={2010}, month={Feb}, pages={309–324} }
 @article{sankrit_williams_borkowski_gaetz_raymond_blair_ghavamian_long_reynolds_2010, title={DUST DESTRUCTION IN A NON-RADIATIVE SHOCK IN THE CYGNUS LOOP SUPERNOVA REMNANT}, volume={712}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/712/2/1092}, abstractNote={We present 24 μm and 70 μm images of a non-radiative shock in the Cygnus Loop supernova remnant, obtained with the Multiband Imaging Photometer for Spitzer on board the Spitzer Space Telescope. The post-shock region is resolved in these images. The ratio of the 70 μm to the 24 μm flux rises from about 14 at a distance 0.′1 behind the shock front to about 22 in a zone 0.′75 further downstream, as grains are destroyed in the hot plasma. Models of dust emission and destruction using post-shock electron temperatures between 0.15 keV and 0.30 keV and post-shock densities, nH∼ 2.0 cm−3, predict flux ratios that match the observations. Non-thermal sputtering (i.e., sputtering due to bulk motion of the grains relative to the gas) contributes significantly to the dust destruction under these shock conditions. From the model calculations, we infer that about 35% by mass of the grains are destroyed over a 0.14 pc region behind the shock front.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Sankrit, Ravi and Williams, Brian J. and Borkowski, Kazimierz J. and Gaetz, Terrance J. and Raymond, John C. and Blair, William P. and Ghavamian, Parviz and Long, Knox S. and Reynolds, Stephen P.}, year={2010}, month={Apr}, pages={1092–1099} }
 @article{borkowski_reynolds_green_hwang_petre_krishnamurthy_willett_2010, title={RADIOACTIVE SCANDIUM IN THE YOUNGEST GALACTIC SUPERNOVA REMNANT G1.9+0.3}, volume={724}, ISSN={["2041-8213"]}, DOI={10.1088/2041-8205/724/2/l161}, abstractNote={We report the discovery of thermal X-ray emission from the youngest Galactic supernova remnant G1.9+0.3, from a 237 ks Chandra observation. We detect strong Kα lines of Si, S, Ar, Ca, and Fe. In addition, we detect a 4.1 keV line with 99.971% confidence which we attribute to 44Sc, produced by electron capture from 44Ti. Combining the data with our earlier Chandra observation allows us to detect the line in two regions independently. For a remnant age of 100 yr, our measured total line strength indicates synthesis of (1–7) × 10−5 M☉ of 44Ti, in the range predicted for both Type Ia and core-collapse supernovae (SNe), but somewhat smaller than the 2 × 10−4 M☉ reported for Cas A. The line spectrum indicates supersolar abundances. The Fe emission has a width of about 28,000 km s−1, consistent with an age of ∼100 yr and with the inferred mean shock velocity of 14,000 km s−1 deduced assuming a distance of 8.5 kpc. Most thermal emission comes from regions of lower X-ray but higher radio surface brightness. Deeper observations should allow more detailed spatial mapping of 44Sc, with significant implications for models of nucleosynthesis in Type Ia SNe.}, number={2}, journal={ASTROPHYSICAL JOURNAL LETTERS}, author={Borkowski, Kazimierz J. and Reynolds, Stephen P. and Green, David A. and Hwang, Una and Petre, Robert and Krishnamurthy, Kalyani and Willett, Rebecca}, year={2010}, month={Dec}, pages={L161–L165} }
 @article{katsuda_petre_mori_reynolds_long_winkler_tsunemi_2010, title={STEADY X-RAY SYNCHROTRON EMISSION IN THE NORTHEASTERN LIMB OF SN 1006}, volume={723}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/723/1/383}, abstractNote={We investigate time variations and detailed spatial structures of X-ray synchrotron emission in the northeastern limb of SN 1006, using two Chandra observations taken in 2000 and 2008. We extract spectra from a number of small (∼10″) regions. After taking account of proper motion and isolating the synchrotron from the thermal emission, we study time variations in the synchrotron emission in the small regions. We find that there are no regions showing strong flux variations. Our analysis shows an apparent flux decline in the overall synchrotron flux of ∼4% at high energies, but we suspect that this is mostly a calibration effect, and that flux is actually constant to ∼1%. This is much less than the variation found in other remnants where it was used to infer magnetic-field strengths up to 1 mG. We attribute the lack of variability to the smoothness of the synchrotron morphology, in contrast to the small-scale knots found to be variable in other remnants. The smoothness is to be expected for a Type Ia remnant encountering uniform material. Finally, we find a spatial correlation between the flux and the cutoff frequency in synchrotron emission. The simplest interpretation is that the cutoff frequency depends on the magnetic-field strength. This would require that the maximum energy of accelerated electrons is not limited by synchrotron losses, but by some other effect. Alternatively, the rate of particle injection and acceleration may vary due to some effect not yet accounted for, such as a dependence on shock obliquity.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Katsuda, Satoru and Petre, Robert and Mori, Koji and Reynolds, Stephen P. and Long, Knox S. and Winkler, P. Frank and Tsunemi, Hiroshi}, year={2010}, month={Nov}, pages={383–392} }
 @article{borkowski_blondin_reynolds_2009, title={CIRCUMSTELLAR SHELLS IN ABSORPTION IN TYPE Ia SUPERNOVAE}, volume={699}, ISSN={["2041-8213"]}, DOI={10.1088/0004-637X/699/2/L64}, abstractNote={Progenitors of Type Ia supernovae (SNe) have been predicted to modify their ambient circumstellar (CSM) and interstellar environments through the action of their powerful winds. While there is X-ray and optical evidence for circumstellar interaction in several remnants of Type Ia SNe, widespread evidence for such interaction in Type Ia SNe themselves has been lacking. We consider prospects for the detection of CSM shells that have been predicted to be common around Type Ia SNe. Such shells are most easily detected in Na I absorption lines. Variable (declining) absorption is expected to occur soon after the explosion, primarily during the SN rise time, for shells located within ∼1–10 pc of a SN. The distance of the shell from the SN can be determined by measuring the timescale for line variability.}, number={2}, journal={ASTROPHYSICAL JOURNAL LETTERS}, publisher={IOP Publishing}, author={Borkowski, Kazimierz J. and Blondin, John M. and Reynolds, Stephen P.}, year={2009}, month={Jul}, pages={L64–L67} }
 @article{rea_mclaughlin_gaensler_slane_stella_reynolds_burgay_israel_possenti_chatterjee_et al._2009, title={DISCOVERY OF EXTENDED X-RAY EMISSION AROUND THE HIGHLY MAGNETIC RRAT J1819-1458}, volume={703}, ISSN={["2041-8213"]}, DOI={10.1088/0004-637X/703/1/L41}, abstractNote={We report on the discovery of extended X-ray emission around the high magnetic field rotating radio transient J1819–1458. Using a 30 ks Chandra ACIS-S observation, we found significant evidence for extended X-ray emission with a peculiar shape: a compact region out to ∼5.″5, and more diffuse emission extending out to ∼13″ from the source. The most plausible interpretation is a nebula somehow powered by the pulsar, although the small number of counts prevents a conclusive answer on the nature of this emission. RRAT J1819–1458's spin-down energy loss rate ( erg s−1) is much lower than that of other pulsars with observed spin-down-powered pulsar wind nebulae (PWNe), and implies a rather high X-ray efficiency of at converting spin-down power into the PWN X-ray emission. This suggests the need of an additional source of energy rather than the spin-down power alone, such as the high magnetic energy of this source. Furthermore, this Chandra observation allowed us to refine the positional accuracy of RRAT J1819–1458 to a radius of ∼0.″3, and confirms the presence of X-ray pulsations and the ∼1 keV absorption line, previously observed in the X-ray emission of this source.}, number={1}, journal={ASTROPHYSICAL JOURNAL LETTERS}, author={Rea, N. and McLaughlin, M. A. and Gaensler, B. M. and Slane, P. O. and Stella, L. and Reynolds, S. P. and Burgay, M. and Israel, G. L. and Possenti, A. and Chatterjee, S. and et al.}, year={2009}, month={Sep}, pages={L41–L45} }
 @article{babson_reynolds_bjorkquist_griffiths_2009, title={Hidden momentum, field momentum, and electromagnetic impulse}, volume={77}, ISSN={["0002-9505"]}, DOI={10.1119/1.3152712}, abstractNote={Electromagnetic fields carry energy, momentum, and angular momentum. The momentum density, ϵ0(E×B), accounts (among other things) for the pressure of light. But even static fields can carry momentum, and this would appear to contradict a general theorem that the total momentum of a closed system is zero if its center of energy is at rest. In such cases, there must be some other (nonelectromagnetic) momenta that cancel the field momentum. What is the nature of this “hidden momentum” and what happens to it when the electromagnetic fields are turned off?}, number={9}, journal={AMERICAN JOURNAL OF PHYSICS}, author={Babson, David and Reynolds, Stephen P. and Bjorkquist, Robin and Griffiths, David J.}, year={2009}, month={Sep}, pages={826–833} }
 @article{reynolds_2009, title={SYNCHROTRON-LOSS SPECTRAL BREAKS IN PULSAR-WIND NEBULAE AND EXTRAGALACTIC JETS}, volume={703}, ISSN={["0004-637X"]}, DOI={10.1088/0004-637X/703/1/662}, abstractNote={Flows of synchrotron-emitting material can be found in several astrophysical contexts, including extragalactic jets and pulsar-wind nebulae (PWNe). For X-ray synchrotron emission, flow times are often longer than electron radiative lifetimes, so the effective source size at a given X-ray energy is the distance electrons radiating at that energy can convect before they burn off. Since synchrotron losses vary strongly with electron energy, the source size drops with increasing X-ray energy, resulting in a steepening of the synchrotron spectrum. For homogeneous sources, this burnoff produces the well known result of a steepening by 0.5 in the source's integrated spectral index. However, for inhomogeneous sources, different amounts of steepening are possible. I exhibit a simple phenomenological picture of an outflow of relativistic electrons with bulk nonrelativistic flow speed, with transverse flow-tube radius, magnetic field strength, matter density, and flow velocity all varying as different powers of distance from the injection point. For such a picture, I calculate the value of the spectral index above the break as a function of the power-law indices, and show the possible range of steepenings. I show that these simple calculations are confirmed by full integrations of source luminosity, which also include the spectral “bump” below the break from the accumulation of electrons formerly at higher energies. In many cases, extragalactic jets show X-ray synchrotron emission steeper by more than 0.5 than the radio emission; the same phenomenon is exhibited by many PWNe. It is possible that source inhomogeneities are responsible in at least some cases, so that the amount of spectral steepening becomes a diagnostic for source dynamical or geometrical properties.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Reynolds, Stephen P.}, year={2009}, month={Sep}, pages={662–670} }
 @article{katsuda_petre_long_reynolds_winkler_mori_tsunemi_2009, title={THE FIRST X-RAY PROPER-MOTION MEASUREMENTS OF THE FORWARD SHOCK IN THE NORTHEASTERN LIMB OF SN 1006}, volume={692}, DOI={10.1088/0004-637X/692/2/L105}, abstractNote={We report on the first X-ray proper-motion measurements of the nonthermally dominated forward shock in the northeastern limb of SN 1006, based on two Chandra observations taken in 2000 and 2008. We find that the proper motion of the forward shock is about 0.″48 yr−1 and does not vary around the rim within the ∼10% measurement uncertainties. The proper motion measured is consistent with that determined by previous radio observations. The mean expansion index of the forward shock is calculated to be ∼0.54 which matches the value expected based on an evolutionary model of a Type Ia supernova with either a power-law or an exponential ejecta density profile. Assuming pressure equilibrium around the periphery from the thermally dominated northwestern rim to the nonthermally dominated northeastern rim, we estimate the ambient density to the northeast of SN 1006 to be ∼0.085 cm−3.}, number={2}, journal={ASTROPHYSICAL JOURNAL LETTERS}, author={Katsuda, Satoru and Petre, Robert and Long, Knox S. and Reynolds, Stephen P. and Winkler, P. Frank and Mori, Koji and Tsunemi, Hiroshi}, year={2009}, month={Feb}, pages={L105–L108} }
 @article{reynolds_borkowski_green_hwang_harrus_petre_2009, title={X-RAY SPECTRAL VARIATIONS IN THE YOUNGEST GALACTIC SUPERNOVA REMNANT G1.9+0.3}, volume={695}, ISSN={["2041-8205"]}, DOI={10.1088/0004-637X/695/2/L149}, abstractNote={The discovery of the youngest Galactic supernova remnant (SNR) G1.9+0.3 has allowed a look at a stage of SNR evolution never before observed. We analyze the 50 ks Chandra observation with particular regard to spectral variations. The very high column density (NH ∼ 6 × 1022 cm−2) implies that dust scattering is important, and we use a simple scattering model in our spectral analysis. The integrated X-ray spectrum of G1.9+0.3 is well described by synchrotron emission from a power-law electron distribution with an exponential cutoff. Using our measured radio flux and including scattering effects, we find a rolloff frequency of 5.4(3.0, 10.2) × 1017 Hz (hνroll = 2.2 keV). Including scattering in a two-region model gives lower values of νroll by over a factor of 2. Dividing G1.9+0.3 into six regions, we find a systematic pattern in which spectra are hardest (highest νroll) in the bright southeast and northwest limbs of the shell. They steepen as one moves around the shell or into the interior. The extensions beyond the bright parts of the shell have the hardest spectra of all. We interpret the results in terms of dependence of shock acceleration properties on the obliquity angle θBn between the shock velocity and a fairly uniform upstream magnetic field. This interpretation probably requires a Type Ia event. If electron acceleration is limited by synchrotron losses, the spectral variations require obliquity-dependence of the acceleration rate independent of the magnetic-field strength.}, number={2}, journal={ASTROPHYSICAL JOURNAL LETTERS}, author={Reynolds, Stephen P. and Borkowski, Kazimierz J. and Green, David A. and Hwang, Una and Harrus, Ilana and Petre, Robert}, year={2009}, month={Apr}, pages={L149–L153} }
 @article{williams_borkowski_reynolds_raymond_long_morse_blair_ghavamian_sankrit_hendrick_et al._2008, title={EJECTA, DUST, AND SYNCHROTRON RADIATION IN SNR B0540-69.3: A MORE CRAB-LIKE REMNANT THAN THE CRAB}, volume={687}, ISSN={["1538-4357"]}, DOI={10.1086/592139}, abstractNote={We present near- and mid-infrared observations of the pulsar-wind nebula (PWN) SNR B0540–69.3 and its associated supernova remnant made with the Spitzer Space Telescope. We report detections of the PWN with all four IRAC bands, the 24 μm band of MIPS, and the Infrared Spectrograph (IRS). We find no evidence of IR emission from the X-ray/radio shell surrounding the PWN resulting from the forward shock of the supernova blast wave. The flux of the PWN itself is dominated by synchrotron emission at shorter (IRAC) wavelengths, with a warm dust component longward of 20 μm. We show that this dust continuum can be explained by a small amount [~(1–3) × 10−3 M☉] of dust at a temperature of ~50-65 K, heated by the shock wave generated by the PWN being driven into the inner edge of the ejecta. This is evidently dust synthesized in the supernova. We also report the detection of several lines in the spectrum of the PWN and present kinematic information about the PWN as determined from these lines. Kinematics are consistent with previous optical studies of this object. Line strengths are also broadly consistent with what one expects from optical line strengths. We find that lines arise from slow (~20 km s−1) shocks driven into oxygen-rich clumps in the shell swept up by an iron-nickel bubble, which have a density contrast of ~100-200 relative to the bulk of the ejecta, and that faster shocks (~250 km s−1) in the hydrogen envelope are required to heat dust grains to observed temperatures. We infer from estimates of heavy-element ejecta abundances that the progenitor star was likely in the range of 20-25 M☉.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Williams, Brian J. and Borkowski, Kazimierz J. and Reynolds, Stephen P. and Raymond, John C. and Long, Knox S. and Morse, Jon and Blair, William P. and Ghavamian, Parviz and Sankrit, Ravi and Hendrick, Sean P. and et al.}, year={2008}, month={Nov}, pages={1054–1069} }
 @article{reynolds_2008, title={Supernova remnants at high energy}, volume={46}, ISSN={["0066-4146"]}, DOI={10.1146/annurev.astro.46.060407.145237}, abstractNote={ Many shell supernova remnants are now known to radiate synchrotron X-rays. Several objects have also been detected in TeV gamma rays. Nonthermal X-rays and gamma rays can be produced in shell remnants by extremely energetic ions and electrons due to decay of π0 mesons produced in inelastic collisions between ions and thermal gas, or by electron synchrotron, bremsstrahlung, or inverse-Compton radiation. Thus observations at X-ray and gamma-ray wavelengths constrain the process of particle acceleration to high energies in the shock waves of supernova remnants. This review examines the relevant characteristics of Type Ia and core-collapse supernovae, the dynamics of their evolution through the Sedov blast-wave phase, the basic physics of diffusive shock acceleration, and the physics of the relevant radiative processes. It also reviews the current status of observations of shell remnants from X-rays to TeV gamma rays, and summarizes what we can learn about particle acceleration. }, journal={ANNUAL REVIEW OF ASTRONOMY AND ASTROPHYSICS}, author={Reynolds, Stephen P.}, year={2008}, pages={89–126} }
 @article{slane_helfand_reynolds_gaensler_lemiere_wang_2008, title={The infrared detection of the pulsar wind nebula in the galactic supernova remnant 3C 58}, volume={676}, ISSN={["2041-8205"]}, DOI={10.1086/587031}, abstractNote={We present infrared observations of 3C 58 with the Spitzer Space Telescope and the Canada-France-Hawaii Telescope. Using the IRAC camera, we have imaged the entire source, which results in clear detections of the nebula at 3.6 and 4.5 μm. The derived flux values are consistent with extrapolation of the X-ray spectrum to the infrared band, demonstrating that any cooling break in the synchrotron spectrum must occur near the soft X-ray band. We also detect the torus surrounding PSR J0205+6449, the 65 ms pulsar that powers 3C 58. The torus spectrum requires a break between the infrared and X-ray bands, and perhaps multiple breaks. This complex spectrum, which is an imprint of the particles injected into the nebula, has considerable consequences for the evolution of the broadband spectrum of 3C 58. We illustrate these effects and discuss the impact of these observations on the modeling of broadband spectra of pulsar wind nebulae.}, number={1}, journal={ASTROPHYSICAL JOURNAL LETTERS}, author={Slane, P. and Helfand, D. J. and Reynolds, S. P. and Gaensler, B. M. and Lemiere, A. and Wang, Z.}, year={2008}, month={Mar}, pages={L33–L36} }
 @article{reynolds_borkowski_green_hwang_harrus_petre_2008, title={The youngest Galactic supernova remnant: G1.9+0.3}, volume={680}, DOI={10.1086/589570}, abstractNote={Our 50 ks Chandra observation of the small radio supernova remnant (SNR) G1.9+0.3 shows a complete shell structure with strong bilateral symmetry, about 100″ in diameter. The radio morphology is also shell-like, but only about 84″ in diameter, based on observations made in 1985. We attribute the size difference to expansion between 1985 and our Chandra observations of 2007. Expansion is confirmed in comparing radio images from 1985 and 2008. We deduce that G1.9+0.3 is of order 100 years old—the youngest supernova remnant in the Galaxy. Based on a very high absorbing column density of 5.5 × 1022 cm−2, we place G1.9+0.3 near the Galactic center, at a distance of about 8.5 kpc, where the mean remnant radius would be about 2 pc, and the required expansion speed about 14,000 km s−1. The X-ray spectrum is featureless and well described by the exponentially cut off synchrotron model srcut. With the radio flux at 1 GHz fixed at 0.9 Jy, we find a spectral index of 0.65 and a rolloff frequency of 1.4 × 1018 Hz. The implied characteristic rolloff electron energy of about 94(B/10 μ G)−1/2 TeV is the highest ever reported for a shell supernova remnant. It can easily be reached by standard diffusive shock acceleration, given the very high shock velocities; it can be well described by either age-limited or synchrotron-loss-limited acceleration. Not only is G1.9+0.3 the youngest known Galactic remnant, it is also only the fourth Galactic X-ray-synchrotron-dominated shell supernova remnant.}, number={1}, journal={ASTROPHYSICAL JOURNAL LETTERS}, author={Reynolds, Stephen P. and Borkowski, Kazimierz J. and Green, David A. and Hwang, Una and Harrus, Ilana and Petre, Robert}, year={2008}, month={Jun}, pages={L41–L44} }
 @article{reynolds_borkowski_hwang_hughes_badenes_laming_blondin_2007, title={A deep Chandra observation of Kepler's supernova remnant: A type IA event with circumstellar interaction}, volume={668}, ISSN={["1538-4357"]}, DOI={10.1086/522830}, abstractNote={We present initial results of a 750 ks Chandra observation of the remnant of Kepler's supernova of AD 1604. The strength and prominence of iron emission, together with the absence of O-rich ejecta, demonstrate that Kepler resulted from a thermonuclear supernova, even though evidence for circumstellar interaction is also strong. We have analyzed spectra of over 100 small regions, and find that they fall into three classes. (1) The vast majority show Fe L emission between 0.7 and 1 keV and Si and S Kα emission; we associate these with shocked ejecta. A few of these are found at or beyond the mean blast wave radius. (2) A very few regions show solar O/Fe abundance ratios; these we associate with shocked circumstellar medium (CSM). Otherwise O is scarce. (3) A few regions are dominated by continuum, probably synchrotron radiation. Finally, we find no central point source, with a limit ~100 times fainter than the central object in Cas A. The evidence that the blast wave is interacting with CSM may indicate a Ia explosion in a more massive progenitor.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Reynolds, Stephen P. and Borkowski, Kazimierz J. and Hwang, Una and Hughes, John P. and Badenes, Carles and Laming, J. M. and Blondin, J. M.}, year={2007}, month={Oct}, pages={L135–L138} }
 @article{gaensler_mclaughlin_reynolds_borkowski_rea_possenti_israel_burgay_camilo_chatterjee_et al._2007, title={Chandra smells a RRAT}, volume={308}, ISSN={["0004-640X"]}, DOI={10.1007/s10509-007-9352-8}, number={1-4}, journal={ASTROPHYSICS AND SPACE SCIENCE}, author={Gaensler, Bryan M. and McLaughlin, Maura and Reynolds, Stephen and Borkowski, Kazik and Rea, Nanda and Possenti, Andrea and Israel, Gianluca and Burgay, Marta and Camilo, Fernando and Chatterjee, Shami and et al.}, year={2007}, month={Apr}, pages={95–99} }
 @article{reynolds_2007, title={Cosmic-ray acceleration in SN 1006 and other shell supernova remnants}, DOI={10.1143/ptps.169.136}, abstractNote={I review the discovery of synchrotron X-rays from the remnant of the supernova of 1006 AD, and the significance of that discovery for our understanding of particle acceleration in shock waves and the origin of cosmic rays. I survey the current observational and theoretical landscape on these issues.}, number={169}, journal={Progress of Theoretical Physics. Supplement}, author={Reynolds, Stephen}, year={2007}, pages={136–141} }
 @article{morton_slane_borkowski_reynolds_helfand_gaensler_hughes_2007, title={Observations of X-rays and thermal dust emission from the supernova remnant Kes 75}, volume={667}, ISSN={["0004-637X"]}, DOI={10.1086/520496}, abstractNote={We present Spitzer Space Telescope and Chandra X-Ray Observatory observations of the composite Galactic supernova remnant Kes 75 (G29.7-0.3). We use the detected flux at 24 μm and hot gas parameters from fitting spectra from new, deep X-ray observations to constrain models of dust emission, obtaining a dust-to-gas mass ratio Mdust/Mgas ~ 10-3. We find that a two-component thermal model, nominally representing shocked swept-up interstellar or circumstellar material and reverse-shocked ejecta, adequately fits the X-ray spectrum, albeit with somewhat high implied densities for both components. We surmise that this model implies a Wolf-Rayet progenitor for the remnant. We also present infrared flux upper limits for the central pulsar wind nebula.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Morton, Timothy D. and Slane, Patrick and Borkowski, Kazimierz J. and Reynolds, Stephen P. and Helfand, David J. and Gaensler, B. M. and Hughes, John P.}, year={2007}, month={Sep}, pages={219–225} }
 @inproceedings{burgay_mclaughlin_reynolds_2007, title={On the debated nature of Rotating RAdio Transients}, url={http://dx.doi.org/10.1063/1.2774917}, DOI={10.1063/1.2774917}, abstractNote={Rotating RAdio Transients (RRATs) are a newly discovered astronomical phenomenon, characterised by occasional radio bursts occurring at average intervals ranging from minutes to hours. Analysis of the burst arrival times allowed the identification of periodicities, which fall in the range 0.4 to 7 seconds. For three of them also a secular variation in the periodicity has been measured. In view of these rotational parameters the RRATs seem to be rotating neutron stars, albeit with properties very different from the rest of the population. The characteristics of these sources in the radio band and the X‐ray properties of the first detected counterpart to a RRAT are compared with the multiwavelength properties of other classes of neutron stars trying to explain the nature of RRATs and their connections with known types of objects.}, publisher={American Institute of Physics}, author={Burgay, M. and McLaughlin, M. A. and Reynolds, S. P.}, year={2007} }
 @article{blair_ghavamian_long_williams_borkowski_reynolds_sankrit_2007, title={Spitzer space telescope observations of Kepler's supernova remnant: A detailed look at the circumstellar dust component}, volume={662}, ISSN={["1538-4357"]}, DOI={10.1086/518414}, abstractNote={We present 3.6-160 μm infrared images of Kepler's supernova remnant (SN 1604) obtained with the IRAC and MIPS instruments on the Spitzer Space Telescope. We also present MIPS SED low-resolution spectra in the 55-95 μm region. The observed emission in the MIPS 24 μm band shows the entire shell. Emission in the MIPS 70 μm and IRAC 8 μm bands is seen only from the brightest regions of 24 μm emission, which also correspond to the regions seen in optical Hα images. Shorter wavelength IRAC images are increasingly dominated by stars, although faint filaments are discernible. The SED spectrum of shows a faint continuum dropping off to longer wavelengths and confirms that strong line emission does not dominate the mid-IR spectral region. The emission we see is due primarily to warm dust emission from dust heated by the primary blast wave; no excess infrared emission is observed in regions where supernova ejecta are seen in X-rays. We use models of the dust to interpret the observed 70/24 μm ratio and constrain the allowed range of temperatures and densities. We estimate the current mass of dust in the warm dust component to be 5.4 × 10-4 M☉, and infer an original mass of about 3 × 10-3 M☉ before grain sputtering. The MIPS 160 μm band shows no emission belonging to the remnant. We place a conservative but temperature-dependent upper limit on any cold dust component roughly a factor of 10 below the cold dust mass inferred from SCUBA observations. Finally, we comment on issues relevant to the possible precursor star and the supernova type.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Blair, William P. and Ghavamian, Parviz and Long, Knox S. and Williams, Brian J. and Borkowski, Kazimierz J. and Reynolds, Stephen P. and Sankrit, Ravi}, year={2007}, month={Jun}, pages={998–1013} }
 @article{borkowski_hendrick_reynolds_2007, title={X-ray-emitting ejecta of supernova remnant N132D}, volume={671}, ISSN={["2041-8213"]}, DOI={10.1086/524733}, abstractNote={The brightest supernova remnant in the Magellanic Clouds, N132D, belongs to the rare class of oxygen-rich remnants, about a dozen objects that show optical emission from pure heavy-element ejecta. They originate in explosions of massive stars that produce large amounts of O, although only a tiny fraction of that O is found to emit at optical wavelengths. We report the detection of substantial amounts of O at X-ray wavelengths in a recent 100 ks Chandra ACIS observation of N132D. A comparison between subarcsecond-resolution Chandra and Hubble images reveals a good match between clumpy X-ray and optically emitting ejecta on large (but not small) scales. Ejecta spectra are dominated by strong lines of He- and H-like O; they exhibit substantial spatial variations partially caused by patchy absorption within the LMC. Because optical ejecta are concentrated in a 5 pc radius elliptical expanding shell, the detected ejecta X-ray emission also originates in this shell.}, number={1}, journal={ASTROPHYSICAL JOURNAL LETTERS}, author={Borkowski, Kazimierz J. and Hendrick, Sean P. and Reynolds, Stephen P.}, year={2007}, month={Dec}, pages={L45–L48} }
 @article{reynolds_borkowski_hwang_harrus_petre_dubner_2006, title={A new young galactic supernova remnant containing a compact object: g15.9+0.2}, volume={652}, ISSN={["0004-637X"]}, DOI={10.1086/510066}, abstractNote={We identify the radio-emitting shell-type supernova remnant G15.9+0.2 as a relatively young remnant containing an X-ray point source that may be its associated neutron star. The integrated spectrum of the remnant shell obtained from our 30 ks exploratory Chandra observation shows very strong lines that require elevated element abundances from ejecta, in particular of sulfur. A plane-shock model fit gives a temperature kT = 0.9 (0.8, 1.0) keV, an ionization timescale net = 6 (4, 9) × 1010 cm-3 s, and a sulfur abundance of 2.1 (1.7, 2.7) times solar (90% confidence limits). Two-component models with one solar and one enriched component are also plausible, but they are not well constrained by the data. Various estimates give a remnant age of order 103 yr, which would make G15.9+0.2 among the dozen or so youngest remnants in the Galaxy. The sparse point-source spectrum is consistent with either a steep Γ ~ 4 power law or a kT ~ 0.4 keV blackbody. The spectrum is absorbed by a H column density NH ~ 4 × 1022 cm-2 similar to that required for the remnant shell. The implied 2-9.5 keV source luminosity is about 1033 ergs s-1 for an assumed distance of 8.5 kpc consistent with the high absorption column. We suggest that the point source is either a rotation-powered pulsar or a compact central object.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Reynolds, Stephen P. and Borkowski, Kazimierz J. and Hwang, Una and Harrus, Ilana and Petre, Robert and Dubner, Gloria}, year={2006}, month={Nov}, pages={L45–L48} }
 @article{borkowski_hendrick_reynolds_2006, title={Dense, Fe-rich ejecta in supernova remnants DEM L238 and DEM L249: A new class of type Ia supernova?}, volume={652}, ISSN={["1538-4357"]}, DOI={10.1086/508335}, abstractNote={We present observations of two LMC supernova remnants (SNRs), DEM L238 and DEM L249, with the Chandra and XMM-Newton X-ray satellites. Bright central emission, surrounded by a faint shell, is present in both remnants. The central emission has an entirely thermal spectrum dominated by strong Fe L-shell lines, with the deduced Fe abundance in excess of solar and not consistent with the LMC abundance. This Fe overabundance leads to the conclusion that DEM L238 and DEM L249 are remnants of thermonuclear (Type Ia) explosions. The shell emission originates in gas swept up and heated by the blast wave. A standard Sedov analysis implies about 50 M☉ in both swept-up shells, SNR ages between 10,000 and 15,000 yr, low (≲0.05 cm-3) preshock densities, and subluminous explosions with energies of 3 × 1050 ergs. The central Fe-rich supernova ejecta are close to collisional ionization equilibrium. Their presence is unexpected, because standard Type Ia SNR models predict faint ejecta emission with short ionization ages. Both SNRs belong to a previously unrecognized class of Type Ia SNRs characterized by bright interior emission. Denser than expected ejecta and/or a dense circumstellar medium around the progenitors are required to explain the presence of Fe-rich ejecta in these SNRs. Substantial amounts of circumstellar gas are more likely to be present in explosions of more massive Type Ia progenitors. DEM L238, DEM L249, and similar SNRs could be remnants of "prompt" Type Ia explosions with young (~100 Myr old) progenitors.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Borkowski, Kazimierz J. and Hendrick, Sean P. and Reynolds, Stephen P.}, year={2006}, month={Dec}, pages={1259–1267} }
 @article{reynolds_borkowski_gaensler_rea_mclaughlin_possenti_israel_burgay_camilo_chatterjee_et al._2006, title={Discovery of the X-ray counterpart to the Rotating RAdio Transient J1819-1458}, volume={639}, ISSN={["1538-4357"]}, DOI={10.1086/502648}, abstractNote={We present the discovery of the first X-ray counterpart to a Rotating RAdio Transient (RRAT) source. RRAT J1819-1458 is a relatively highly magnetized (B ~ 5 × 1013 G) member of a new class of unusual pulsar-like objects discovered by their bursting activity at radio wavelengths. A Chandra observation of that position revealed a pointlike source, CXOU J181934.1-145804, with a soft spectrum well fit by an absorbed blackbody with NH = 7 × 1021 cm-2, temperature kT = 0.12 ± 0.04 keV, and an unabsorbed flux of ~2 × 10-12 ergs cm-2 s-1 between 0.5 and 8 keV. No optical or infrared (IR) counterparts are visible within 1'' of our X-ray position. The positional coincidence, spectral properties, and lack of an optical/IR counterpart make it highly likely that CXOU J181934.1-145804 is a neutron star and is the same object as RRAT J1819-1458. The source showed no variability on any timescale from the pulse period of 4.26 s up to the 5 day window covered by the observations, although our limits (especially for pulsations) are not particularly constraining. The X-ray properties of CXOU J181934.1-145804, while not yet measured to high precision, are similar to those of comparably aged radio pulsars and are consistent with thermal emission from a cooling neutron star.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Reynolds, SP and Borkowski, KJ and Gaensler, BM and Rea, N and McLaughlin, M and Possenti, A and Israel, G and Burgay, M and Camilo, F and Chatterjee, S and et al.}, year={2006}, month={Mar}, pages={L71–L74} }
 @article{williams_borkowski_reynolds_blair_ghavamian_hendrick_long_points_raymond_sankrit_et al._2006, title={Dust destruction in fast shocks of core-collapse supernova remnants in the large magellanic cloud}, volume={652}, ISSN={["1538-4357"]}, DOI={10.1086/509876}, abstractNote={We report observations with the Multiband Imaging Photometer for Spitzer of four supernova remnants (SNRs) believed to be the result of core-collapse supernovae: N132D (0525-69.6), N49B (0525-66.0), N23 (0506-68.0), and 0453-68.5. All four of these SNRs were detected in whole at 24 μm and in part at 70 μm. Comparisons with Chandra broadband X-ray images show an association of infrared (IR) emission with the blast wave. We attribute the observed IR emission to dust that has been collisionally heated by electrons and ions in the hot, X-ray-emitting plasma, with grain size distributions appropriate for the LMC and the destruction of small grains via sputtering by ions. As with our earlier analysis of Type Ia SNRs, models can reproduce observed 70 μm/24 μm flux ratios only if effects from sputtering are included, destroying small grains. We calculate the mass of dust swept up by the blast wave in these remnants, and we derive a dust-to-gas mass ratio of several times less than the often assumed value of 0.25% for the LMC. We believe that one explanation for this discrepancy could be porous (fluffy) dust grains.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Williams, Brian J. and Borkowski, Kazimierz J. and Reynolds, Stephen P. and Blair, William P. and Ghavamian, Parviz and Hendrick, Sean P. and Long, Knox S. and Points, Sean and Raymond, John C. and Sankrit, Ravi and et al.}, year={2006}, month={Nov}, pages={L33–L36} }
 @article{borkowski_williams_reynolds_blair_ghavamian_sankrit_hendrick_long_raymond_smith_et al._2006, title={Dust destruction in type Ia supernova remnants in the large Magellanic Cloud}, volume={642}, ISSN={["1538-4357"]}, DOI={10.1086/504472}, abstractNote={We present first results from an extensive survey of Magellanic Cloud supernova remnants (SNRs) with the Spitzer Space Telescope. We describe IRAC and MIPS imaging observations at 3.6, 4.5, 5.8, 8, 24, and 70 μm of four Balmer-dominated Type Ia SNRs in the Large Magellanic Cloud (LMC): DEM L71 (0505-67.9), 0509-67.5, 0519-69.0, and 0548-70.4. None was detected in the four short-wavelength IRAC bands, but all four were clearly imaged at 24 μm, and two at 70 μm. A comparison of these images with Chandra broadband X-ray images shows a clear association with the blast wave, and not with internal X-ray emission associated with ejecta. Our observations are well described by one-dimensional shock models of collisionally heated dust emission, including grain size distributions appropriate for the LMC, grain heating by collisions with both ions and electrons, and sputtering of small grains. Model parameters are constrained by X-ray, optical, and far-ultraviolet observations. Our models can reproduce observed 70/24 μm flux ratios only by including sputtering, destroying most grains smaller than 0.03-0.04 μm in radius. We infer total dust masses swept up by the SNR blast waves, before sputtering, on the order of 10-2 M☉, several times less than those implied by a dust-to-gas mass ratio of 0.3% as often assumed for the LMC. Substantial dust destruction has implications for gas-phase abundances.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Borkowski, Kazimierz J. and Williams, Brian J. and Reynolds, Stephen P. and Blair, William P. and Ghavamian, Parviz and Sankrit, Ravi and Hendrick, Sean P. and Long, Knox S. and Raymond, John C. and Smith, R. Chris and et al.}, year={2006}, month={May}, pages={L141–L144} }
 @article{hunt_reynolds_2006, title={Environment and luminosity of supernova remnants}, volume={327}, DOI={10.1002/asna.2006554}, number={5-6}, journal={Astronomische Nachrichten}, author={Hunt, L. K. and Reynolds, Stephen}, year={2006}, pages={448–449} }
 @article{hunt_reynolds_2006, title={Environment and luminosity of supernova remnants}, volume={327}, ISSN={0004-6337 1521-3994}, url={http://dx.doi.org/10.1002/asna.200610554}, DOI={10.1002/asna.200610554}, abstractNote={The explosion of supernovae and the evolution of their remnants (SNRs) accelerate cosmic rays over a vast range of timescales. Magnetic fields can be investigated indirectly through one of the observational signatures of this acceleration, namely radio synchrotron emission. With the aim of better understanding the role of the magnetic field in supernova evolution, we explore the variation of SNR radio luminosities with physical conditions in the surrounding interstellar medium. With a data set that comprises more than 90 individual SNRs in 10 galaxies, and a range of 3000 in ISM density and 104 in radio synchrotron luminosity, we find a significant correlation between the two quantities. The observed trends support the hypothesis that adiabatic compression of magnetic fields by itself is insuf.cient to explain the radio emission of the brighter and more luminous in SNRs. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)}, number={5-6}, journal={Astronomische Nachrichten}, publisher={Wiley}, author={Hunt, L. K. and Reynolds, S. P.}, year={2006}, month={Jun}, pages={448–449} }
 @article{renaud_gros_lebrun_terrier_goldwurm_reynolds_kalemci_2006, title={Imaging extended sources with coded mask telescopes: application to the INTEGRAL IBIS/ISGRI instrument}, volume={456}, ISSN={["1432-0746"]}, DOI={10.1051/0004-6361:20065156}, abstractNote={Context. In coded mask techniques, reconstructed sky images are pseudo-images: they are maps of the correlation between the image recorded on a detector and an array derived from the coded mask pattern. Aims. The INTEGRAL/IBIS telescope provides images where the flux of each detected source is given by the height of the local peak in the correlation map. As such, it cannot provide an estimate of the flux of an extended source. What is needed is intensity sky images giving the flux per solide angle as typically done at other wavelengths. Methods. In this paper, we present the response of the INTEGRAL IBIS/ISGRI coded mask instrument to extended sources. We develop a general method based on analytical calculations in order to measure the intensity and the associated error of any celestial source and validated with Monte-Carlo simulations. Results. We find that the sensitivity degrades almost linearly with the source extent. Analytical formulae are given as well as an easy-to-use recipe for the INTEGRAL user. We check this method on IBIS/ISGRI data but these results are general and applicable to any coded mask telescope.}, number={1}, journal={ASTRONOMY & ASTROPHYSICS}, author={Renaud, M. and Gros, A. and Lebrun, F. and Terrier, R. and Goldwurm, A. and Reynolds, S. and Kalemci, E.}, year={2006}, month={Sep}, pages={389–U76} }
 @article{kalemci_boggs_milne_reynolds_2006, title={Searching for annihilation radiation from SN 1006 with SPI on INTEGRAL}, volume={640}, ISSN={["1538-4357"]}, DOI={10.1086/503289}, abstractNote={Historical Type Ia supernovae are a leading candidate as the source of positrons observed through diffuse annihilation emission in the Galaxy. However, a search for annihilation emission from individual Type Ia supernovae was not possible before the improved sensitivity of INTEGRAL. The total 511 keV annihilation flux from individual SNe Ia, as well as their contribution to the overall diffuse emission, depends critically on the escape fraction of positrons produced in 56Co decays. Late optical light curves suggest that this fraction may be as high as 5%. We have searched for positron annihilation radiation from the historical Type Ia supernova SN 1006 using the SPI instrument on INTEGRAL. We did not detect significant 511 keV line emission, with a 3 σ flux upper limit of 0.59 × 10-4 photons cm-2 s-1 for ~1 Ms exposure time, assuming a FWHM of 2.5 keV. This upper limit corresponds to a 7.5% escape fraction, 50% higher than the expected 5% escape scenario, and rules out the possibility that Type Ia supernovae produce all of the positrons in the Galaxy (~12% escape fraction), if the mean positron lifetime is less than 105 yr. Future observations with INTEGRAL will provide stronger limits on the escape fraction of positrons, the mean positron lifetime, and the contribution of Type Ia supernovae to the overall positron content of the Galaxy.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Kalemci, E and Boggs, SE and Milne, PA and Reynolds, SP}, year={2006}, month={Mar}, pages={L55–L57} }
 @article{kalemci_reynolds_boggs_lund_chenevez_renaud_rho_2006, title={X-ray observations of SN 1006 with INTEGRAL}, volume={644}, ISSN={["1538-4357"]}, DOI={10.1086/503377}, abstractNote={The remnant of the supernova of 1006 AD, the remnant first showing evidence for the presence of X-ray synchrotron emission from shock-accelerated electrons, was observed for ~1000 ks with INTEGRAL in order to study electron acceleration to very high energies. The aim of the observation was to characterize the synchrotron emission and attempt to detect nonthermal bremsstrahlung using the combination of IBIS and JEM-X spatial and spectral coverage. The source was detected with JEM-X between the 2.4 and 8.4 keV bands and was not detected with either ISGRI or SPI above 20 keV. The ISGRI upper limit is about a factor of 4 above current model predictions, but confirms the presence of steepening in the power law extrapolated from lower energies (<4 keV).}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Kalemci, E. and Reynolds, S. P. and Boggs, S. E. and Lund, N. and Chenevez, J. and Renaud, M. and Rho, J.}, year={2006}, month={Jun}, pages={274–278} }
 @article{hendrick_reynolds_borkowski_2005, title={An Fe-Ni Bubble in the Small Magellanic Cloud Supernova Remnant B0049-73.6}, volume={622}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1086/429862}, DOI={10.1086/429862}, abstractNote={We present observations with the Chandra X-Ray Observatory of the supernova remnant (SNR) B0049-73.6 in the Small Magellanic Cloud (SMC). The remnant shows outer emission from swept-up SMC gas and a bright ejecta-dominated ring in the interior. The X-ray spectrum of the outer shell shows normal SMC abundances and allows us to estimate the current blast wave speed at 570 km s-1 from Sedov models. The swept-up mass is equal to 170 M⊙, the SNR age is 14,000 yr, and the explosion energy is 8.1 × 1050 ergs. The brightest parts of the inner ring are dominated by O- and Ne-rich heavy-element ejecta. B0049-73.6 is thus a remnant of a core-collapse explosion. More diffuse interior ejecta emission shows less prominent O and Ne lines. The total mass of O within the heavy-element ejecta is about 0.3 M⊙. The filling fraction of the O-rich gas is less than 1%, and its average ionization timescale of 5 × 1010 cm-3 s suggests that the observed emission is dominated by relatively dense (ne ∼ 0.2 cm-3) ejecta. The bright inner ring most likely is composed of the innermost ejecta swept up by a central "nickel bubble" resulting from radioactive energy input from freshly synthesized 56Ni. This is in a qualitative agreement with theoretical predictions of such centrally located Fe-Ni bubbles, and it confirms the important role of the Ni bubble effect in determining the innermost ejecta structure in core-collapse supernovae.}, number={2}, journal={The Astrophysical Journal}, publisher={IOP Publishing}, author={Hendrick, Sean P. and Reynolds, Stephen P. and Borkowski, Kazimierz J.}, year={2005}, month={Mar}, pages={L117–L120} }
 @article{teegarden_watanabe_jean_knodlseder_lonjou_roques_skinner_ballmoos_weidenspointner_bazzano_et al._2005, title={INTEGRAL SPI limits on electron-positron annihilation radiation from the Galactic plane}, volume={621}, ISSN={["1538-4357"]}, DOI={10.1086/426859}, abstractNote={The center of our Galaxy is a known strong source of electron-positron 511 keV annihilation radiation. Thus far, however, there have been no reliable detections of annihilation radiation outside of the central radian of our Galaxy. One of the primary objectives of the INTEGRAL (International Gamma-Ray Astrophysics Laboratory) mission, launched in 2002 October, is the detailed study of this radiation. The Spectrometer on INTEGRAL (SPI) is a high-resolution, coded-aperture gamma-ray telescope with an unprecedented combination of sensitivity, angular resolution, and energy resolution. We report results from the first 10 months of observation. During this period a significant fraction of the observing time was spent in or near the Galactic plane. No positive annihilation flux was detected outside of the central region (∣ l ∣ > 40°) of our Galaxy. In this paper we describe the observations and data analysis methods and give limits on the 511 keV flux.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Teegarden, BJ and Watanabe, K and Jean, P and Knodlseder, J and Lonjou, V and Roques, JP and Skinner, GK and Ballmoos, P and Weidenspointner, G and Bazzano, A and et al.}, year={2005}, month={Mar}, pages={296–300} }
 @article{lazendic_slane_gaensler_reynolds_plucinsky_hughes_2004, title={A high-resolution study of nonthermal radio and X-ray emission from supernova remnant G347.3-0.5}, volume={602}, ISSN={["0004-637X"]}, DOI={10.1086/380956}, abstractNote={G347.3-0.5 is one of three shell-type supernova remnants (SNRs) in the Galaxy whose X-ray spectrum is dominated by nonthermal emission. This puts G347.3-0.5 in the small but growing class of SNRs for which the X-ray emission reveals directly the presence of extremely energetic electrons accelerated by the SNR shock. We have obtained new high-resolution X-ray and radio data on G347.3-0.5 using the Chandra X-Ray Observatory and the Australia Telescope Compact Array (ATCA), respectively. The bright northwestern peak of the SNR seen in ROSAT and ASCA images is resolved with Chandra into bright filaments and fainter diffuse emission. These features show good correspondence with the radio morphological structure, providing strong evidence that the same population of electrons is responsible for the synchrotron emission in both bands in this part of the remnant. Spectral index information from both observations is presented. We found significant difference in photon index value between bright and faint regions of the SNR shell. Spectral properties of these regions support the notion that efficient particle acceleration is occurring in the bright SNR filaments. We report the detection of linear radio polarization toward the SNR, which is most ordered at the northwestern shell where particle acceleration is presumably occurring. Using our new Chandra and ATCA data, we model the broadband emission from G347.3-0.5 with the synchrotron and inverse Compton mechanisms and discuss the conditions under which this is a plausible scenario.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Lazendic, JS and Slane, PO and Gaensler, BM and Reynolds, SP and Plucinsky, PP and Hughes, JP}, year={2004}, month={Feb}, pages={271–285} }
 @article{reynolds_2004, title={Microphysics of shock acceleration from observations of X-ray synchrotron emission from supernova remnants}, volume={33}, ISBN={["*************"]}, ISSN={["0273-1177"]}, DOI={10.1016/j.asr.2003.02.013}, abstractNote={Recent observations of non-thermal X-rays from supernova remnants have been attributed to synchrotron radiation from the loss-steepened tail of a non-thermal distribution of electrons accelerated at the remnant blast wave. In the test-particle limit of diffusive shock acceleration, in which the energy in shock-accelerated particles is unimportant, the slope of a shock-accelerated power-law is independent of the diffusion coefficient κ and on how κ depends on particle energy. However, the maximum energy to which particles can be accelerated depends on the rate of acceleration and that does depend on the energy-dependence of the diffusion coefficient. If the time to accelerate an electron from thermal energies to energy E≫mec2 is τ(E) and if κ∝Eβ, then τ(E)∝Eβ in parallel shocks and τ∝E2−β in perpendicular shocks. Most work on shock acceleration has made the plausible assumption that κ∝rg (where rg is the particle gyroradius), so that β=1 at relativistic energies, implying a particular (wavelength-independent) spectrum of MHD turbulence, where Kolmogorov or Kraichnan spectra might be more physically plausible. I derive the β dependence of the maximum electron energy resulting from limitations due to radiative (synchrotron and inverse-Compton) losses and to finite remnant age (or size). I then exhibit calculations of synchrotron X-ray spectra, and model images, for supernova remnants as a function of β and compare to earlier β=1 results. Spectra can be considerably altered for β<1 and images are dramatically different for values of β corresponding to Kolmogorov or Kraichnan spectra of turbulence. The predicted images are quite unlike observed remnants, suggesting that the turbulence near SNRs is generated by the high-energy particles themselves.}, number={4}, journal={HIGH-ENERGY STUDIES OF SUPERNOVA REMNANTS AND NEUTRON STARS}, author={Reynolds, SP}, year={2004}, pages={461–465} }
 @article{dyer_reynolds_borkowski_2004, title={Separating thermal and nonthermal X-rays in supernova remnants. II. Spatially resolved fits to SN 1006 AD}, volume={600}, ISSN={["1538-4357"]}, DOI={10.1086/380093}, abstractNote={We present a spatially resolved spectral analysis of full ASCA observations of the remnant of the supernova of A.D. 1006. This remnant shows both nonthermal X-ray emission from bright limbs, generally interpreted as synchrotron emission from the loss-steepened tail of the nonthermal electron population also responsible for radio emission, and thermal emission from elsewhere in the remnant. In earlier work, we showed that the spatially integrated spectrum was well described by a theoretical synchrotron model in which shock acceleration of electrons was limited by escape, in combination with thermal models indicating high levels of iron from ejecta. Here we use new spatially resolved subsets of the earlier theoretical nonthermal models for the analysis. We find that emission from the bright limbs remains well described by those models and refine the values for the characteristic break frequency. We show that differences between the northeast and southwest nonthermal limbs are small, too small to account easily for the presence of the northeast limb, but not the southwest, in TeV γ-rays. Comparison of spectra of the nonthermal limbs and other regions confirms that simple cylindrically symmetric nonthermal models cannot describe the emission, and we put limits on nonthermal contributions to emission from the center and the northwest and southeast limbs. We can rule out solar-abundance models in all regions, finding evidence for elevated abundances. However, more sophisticated models will be required to accurately characterize these abundances.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Dyer, KK and Reynolds, SP and Borkowski, KJ}, year={2004}, month={Jan}, pages={752–768} }
 @article{long_reynolds_raymond_winkler_dyer_petre_2003, title={Chandra CCD imagery of the northeast and northwest limbs of SN 1006}, volume={586}, ISSN={["0004-637X"]}, DOI={10.1086/367832}, abstractNote={We present deep images of the SN 1006 remnant (G327.6+14.6) with the ACIS instrument on the Chandra X-Ray Observatory. Two regions have been observed, the synchrotron-dominated northeast limb and the thermally dominated northwest limb, as well as a substantial portion of the interior of the remnant shell. The brightest features in the X-ray images correspond closely to radio features in the northeast and to Balmer-dominated filaments in the northwest. The spectra of the brighter filaments in the northeast are harder, with less prominent line emission than those in the northwest. In addition to highly elongated filaments, both images show enhanced clumps of emission well inside of the shock front that appear to be dominated by emission from oxygen. These probably arise from shock-heated ejecta, based on analysis of their X-ray spectra. We find no firm evidence for a halo of X-ray emission outside the shock to the northeast, as predicted by the Fermi shock-acceleration picture, in which relativistic electrons should be diffusing ahead of the shock. Our limits on upstream emission are less than 1.5% of the postshock levels in regions where the supernova remnant is brightest. This strongly suggests that the bright rims are flattened sheets nearly perpendicular to the plane of the sky and that the magnetic field strength jumps at the shock by a factor significantly larger than 4, as has been proposed if the shock puts significant energy into accelerating nonthermal ions. The spectra obtained of the northwest rim are all dominated by the helium-like ions of O, Ne, Mg, and Si expected from shocks with ionization (net) parameters of order 100 cm-3 yr and electron temperatures of 0.5-1 keV, far lower than the postshock ion temperature implied by estimates of the shock speed obtained from the shape of the Hα line.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Long, KS and Reynolds, SP and Raymond, JC and Winkler, PF and Dyer, KK and Petre, R}, year={2003}, month={Apr}, pages={1162–1178} }
 @article{olbert_keohane_arnaud_dyer_reynolds_safi-harb_2003, title={Chandra detection of a pulsar wind nebula associated with supernova remnant 3C 396}, volume={592}, ISSN={["0004-637X"]}, DOI={10.1086/377348}, abstractNote={We present a 100 ks observation of the Galactic supernova remnant 3C 396 (G39.2-0.3) with the Chandra X-Ray Observatory that we compare with a 20 cm map of the remnant from the Very Large Array. In the Chandra images, a nonthermal nebula containing an embedded pointlike source is apparent near the center of the remnant that we interpret as a synchrotron pulsar wind nebula surrounding a yet undetected pulsar. From the 2-10 keV spectrum for the nebula [NH = (5.3 ± 0.9) × 1022 cm-2, Γ = 1.5 ± 0.3] we derive an unabsorbed X-ray flux of SX = 1.62 × 10-12 ergs cm-2 s-1, and from this we estimate the spin-down power of the neutron star to be = 7.2 × 1036 ergs s-1. The central nebula is morphologically complex, showing bent, extended structure. The radio and X-ray shells of the remnant correlate poorly on large scales, particularly on the eastern half of the remnant, which appears very faint in X-ray images. At both radio and X-ray wavelengths, the western half of the remnant is substantially brighter than the eastern half.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Olbert, CM and Keohane, JW and Arnaud, KA and Dyer, KK and Reynolds, SP and Safi-Harb, S}, year={2003}, month={Jul}, pages={L45–L48} }
 @article{gaensler_hendrick_reynolds_borkowski_2003, title={Discovery of a new pulsar wind nebula in the Large Magellanic Cloud}, volume={594}, ISSN={["0004-637X"]}, DOI={10.1086/378687}, abstractNote={We present new high-resolution radio and X-ray observations of the supernova remnant (SNR) B0453-685 in the Large Magellanic Cloud, carried out with the Australia Telescope Compact Array and the Chandra X-Ray Observatory, respectively. Embedded in the SNR shell is a compact central nebula producing both flat-spectrum polarized radio emission and nonthermal X-rays; we identify this source as a pulsar wind nebula (PWN) powered by an unseen central neutron star. We present a new approach by which the properties of a SNR and PWN can be used to infer upper limits on the initial spin period and surface magnetic field of the unseen pulsar, and we conclude that this star was an initial rapid rotator with current properties similar to those of the Vela pulsar. As is the case for other similarly aged sources, there is currently an interaction taking place between the PWN and the SNR's reverse shock.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Gaensler, BM and Hendrick, SP and Reynolds, SP and Borkowski, KJ}, year={2003}, month={Sep}, pages={L111–L114} }
 @article{hendrick_borkowski_reynolds_2003, title={Ejecta detection in middle-aged large magellanic cloud supernova remnants 0548-70.4 and 0534-69.9}, volume={593}, ISSN={["0004-637X"]}, DOI={10.1086/376356}, abstractNote={We have observed supernova remnants 0548-70.4 and 0534-69.9 in the Large Magellanic Cloud (LMC) with the Chandra X-Ray Observatory and report on the X-ray spectral analysis. Our images of 0548-70.4 and 0534-69.9 show bright central regions as well as brightened limbs. The X-ray spectra from the central regions exhibit enhanced metal abundances, in significant contrast to limb spectra, which show abundances consistent with the LMC interstellar medium (ISM). Considering the relatively old ages (~10,000 yr), these supernova remnants might be assumed to be in the Sedov phase, in which the X-ray spectra would be dominated by swept-up ISM material. The detection of high abundances in these old remnants is therefore surprising. Spectra from the limb regions were analyzed with Sedov models. The results were then used to account for blast wave emission seen in projection toward the central region and were added to a plane-parallel shock model for the reverse shock in the ejecta. We find elevated levels of iron, oxygen, magnesium, silicon, and sulfur in the bright central regions of each remnant. We introduce a new X-ray spectral shock model appropriate for heavy-element-dominated plasmas, in which electrons liberated by successive ionizations dominate the electron pool and modify the electron temperature profile. With this model, we find reverse-shock speeds of 420 km s-1 for 0548-70.4, 500 km s-1 for the northeast central region of 0534-69.9, and 360 km s-1 for its south central region. The elemental abundances favor a Type Ia supernova origin for both 0548-70.4 and 0534-69.9.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Hendrick, SP and Borkowski, KJ and Reynolds, SP}, year={2003}, month={Aug}, pages={370–376} }
 @article{rho_reynolds_reach_jarrett_allen_2003, title={Near-infrared synchrotron emission from Cassiopeia A}, volume={592}, ISSN={["0004-637X"]}, DOI={10.1086/375564}, abstractNote={Recent high-energy observations of Cassiopeia A suggest the presence of synchrotron radiation, implying acceleration of cosmic rays by young supernova remnants. We detect synchrotron emission from Cas A in the near-infrared using Two Micron All-Sky Survey (2MASS) and Palomar 200 inch (5.1 m) PFIRCAM observations. The remnant is detected in the J, H, and Ks bands using 2MASS: the Ks band is the brightest, H is moderate, and J is faint. In the J and H bands, bright [Fe II] lines (1.24 and 1.64 μm) are detected spectroscopically. The Palomar observations include Ks-continuum, narrowband 1.64 μm (centered on [Fe II]) and 2.12 μm [centered on H2 (1-0)] images. While the narrowband 1.64 μm image shows filamentary and knotty structures, similar to the optical image, the Ks image shows a relatively smooth, diffuse shell, remarkably similar to the radio image. The H2 image is identical to the Ks-continuum image, with surface brightness reduced as expected for the ratio of filter bandwidths, showing no contribution of H2 lines to the Ks-band image. The broadband near-infrared fluxes of Cas A are generally consistent with, but a few tens of percent higher than, an extrapolation of the radio fluxes. The hardening to higher frequencies is possibly due to nonlinear shock acceleration and/or spectral index variation across the remnant. We show evidence of spectral index variation across Cas A using the "spectral tomography" technique. The presence of near-infrared synchrotron radiation requires the rolloff frequency to be higher than 1.5 × 1014 Hz, implying that electrons are accelerated to energies of at least E = 0.3B ergs, or 0.2 TeV. The morphological similarity in diffuse emission between the radio and Ks-band images implies that synchrotron losses are not dominant, or we would expect to see a greater concentration in knots. We also show that dust continuum is not significant in the near-infrared emission of Cas A. Our observations show unambiguous evidence that the near-infrared Ks-band emission of Cas A is from synchrotron emission by accelerated cosmic-ray electrons.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Rho, JH and Reynolds, SP and Reach, WT and Jarrett, TH and Allen, GE}, year={2003}, month={Jul}, pages={299–310} }
 @article{rho_dyer_borkowski_reynolds_2002, title={X-ray synchrotron-emitting Fe-rich ejecta in supernova remnant RCW 86}, volume={581}, ISSN={["0004-637X"]}, DOI={10.1086/344248}, abstractNote={Supernova remnants may exhibit both thermal and nonthermal X-ray emission. In a previous study with ASCA data, we found that the middle-aged supernova remnant RCW 86 showed evidence for both processes, and we predicted that observations with much higher spatial resolution would distinguish harder X-rays, which we proposed were primarily synchrotron emission, from softer, thermal X-rays. Here we describe Chandra observations that amply confirm our predictions. Striking differences in the morphology of X-rays below 1 keV and above 2 keV point to a different physical origin. Hard X-ray emission is correlated fairly well with the edges of regions of radio emission, suggesting that these are the locations of shock waves at which both short-lived X-ray-emitting electrons and longer lived radio-emitting electrons are accelerated. Soft X-rays are spatially well correlated with optical emission from nonradiative shocks, which are almost certainly portions of the outer blast wave. These soft X-rays are well fitted with simple thermal plane-shock models. Harder X-rays show Fe Kα emission and are well described with a similar soft thermal component, but a much stronger synchrotron continuum dominating above 2 keV, and a strong Fe Kα line. Quantitative analysis of this line and the surrounding continuum shows that it cannot be produced by thermal emission from a cosmic-abundance plasma; the ionization time is too short, as shown by both the low centroid energy (6.4 keV) and the absence of oxygen lines below 1 keV. Instead, a model of a plane shock in Fe-rich ejecta, with a synchrotron continuum, provides a natural explanation. This requires that reverse shocks in ejecta be accelerating electrons to energies of order 50 TeV. We show that maximum energies of this order can be produced by radiation-limited diffusive shock acceleration at the reverse shocks. In the Appendix, we demonstrate that an explanation of the continuum as being due to nonthermal bremsstrahlung is unlikely.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Rho, JH and Dyer, KK and Borkowski, KJ and Reynolds, SP}, year={2002}, month={Dec}, pages={1116–1131} }
 @article{blondin_borkowski_reynolds_2001, title={Dynamics of Fe bubbles in young supernova remnants}, volume={557}, ISSN={["0004-637X"]}, DOI={10.1086/321674}, abstractNote={Observations of core-collapse supernovae (SNe) have revealed the presence of extensive mixing of radioactive material in SN ejecta. The mixing of radioactive material, mostly freshly synthesized Ni, is not complete, which leads to a two-phase SN ejecta structure. The low-density phase consists of Fe bubbles, created by the energy input from radioactive Co and Ni, surrounded by compressed high-density metal-rich ejecta. We report on the theoretical investigation of supernova remnant (SNR) dynamics with the two-phase SN ejecta. We first present three-dimensional hydrodynamic simulations of a single Fe bubble immersed in an outer ejecta envelope. We then consider randomly distributed Fe bubbles with an average volume filling fraction of 1/2. We find that the presence of Fe bubbles leads to vigorous turbulence and mixing of Fe with other heavy elements and with the ambient normal-abundance gas. The turbulent energy can be an order of magnitude larger than in the case of smooth ejecta. A significant fraction of the shocked ejecta is found in narrow filaments and clumps moving with radial velocities larger than the velocity of the forward shock. Observational consequences of the two-phase ejecta on SNR X-ray spectra and images are briefly mentioned.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Blondin, JM and Borkowski, KJ and Reynolds, SP}, year={2001}, month={Aug}, pages={782–791} }
 @misc{muller_connel_decourchelle_mewaldt_reynolds_strong_volk_wiedenbeck_2001, title={Key measurements in the future - Working group report}, volume={99}, ISSN={["1572-9672"]}, DOI={10.1023/A:1013877922633}, number={1-4}, journal={SPACE SCIENCE REVIEWS}, author={Muller, D and Connel, JJ and Decourchelle, A and Mewaldt, R and Reynolds, S and Strong, A and Volk, H and Wiedenbeck, M}, year={2001}, month={Oct}, pages={353–373} }
 @article{hendrick_reynolds_2001, title={Maximum energies of shock-accelerated electrons in Large Magellanic Cloud supernova remnants}, volume={559}, ISSN={["1538-4357"]}, DOI={10.1086/322341}, abstractNote={Some supernova remnant X-ray spectra show evidence for synchrotron emission from the extension of the electron spectrum that produces radio synchrotron emission. For any remnant, if the extrapolated radio flux exceeds the observed X-ray flux, thermal or nonthermal, a roll-off of the relativistic electron energy distribution must occur below X-ray-emitting energies. We have studied the X-ray emission from 11 remnants in the Large Magellanic Cloud to constrain this roll-off energy. We assume that the electron distribution is a power law with an exponential cutoff at some Emax and radiates in a uniform magnetic field. If the radio flux and spectral index are known, this simple model for the synchrotron contribution depends on only one parameter that relates directly to Emax. Here we have modeled the X-ray spectra by adding a component for thermal radiation of a Sedov blast wave to the synchrotron model. For all 11 supernova remnants in this sample, the limits for Emax range between 10 and 80 TeV (for a mean magnetic field of 10 μG). This result is similar to a study of Galactic remnants in which 13 out of 14 objects had limits between 20 and 80 TeV. We interpret Emax in the context of shock acceleration theories. Better data and models should allow either firm detections of nonthermal components or more restrictive limits on Emax.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Hendrick, SP and Reynolds, SP}, year={2001}, month={Oct}, pages={903–908} }
 @article{dyer_reynolds_borkowski_allen_petre_2001, title={Separating thermal and nonthermal X-rays in supernova remnants. I. Total fits to SN 1006 AD}, volume={551}, ISSN={["0004-637X"]}, DOI={10.1086/320085}, abstractNote={The remnant of SN 1006 has an X-ray spectrum dominated by nonthermal emission, and pre-ASCA observations were well described by a synchrotron calculation with electron energies limited by escape. We describe the results of a much more stringent test: fitting spatially integrated ASCA GIS (0.6-8 keV) and RXTE PCA (3-10 keV) data with a combination of the synchrotron model SRESC newly ported to XSPEC and a new thermal shock model VPSHOCK. The new model can describe the continuum emission above 2 keV well, in both spatial distribution and spectrum. We find that the emission is dominantly nonthermal, with a small but noticeable thermal component: Mg and Si are clearly visible in the integrated spectrum. The synchrotron component rolls off smoothly from the extrapolated radio spectrum, with a characteristic rolloff frequency of 3.1 × 1017 Hz, at which the spectrum has dropped about a factor of 6 below a power-law extrapolation from the radio spectrum. Comparison of TeV observations with new TeV model images and spectra based on the X-ray model fits gives a mean postshock magnetic field strength of about 9 μG, implying (for a compression ratio of 4) an upstream magnetic field of 3 μG, and fixing the current energy content in relativistic electrons at about 7 × 1048 ergs, resulting in a current electron-acceleration efficiency of about 5%. This total energy is about 100 times the energy in the magnetic field. The X-ray fit also implies that electrons escape ahead of the shock above an energy of about 30 TeV. This escape could result from an absence of scattering magnetohydrodynamic waves above a wavelength of about 1017 cm. Our results indicate that joint thermal and nonthermal fitting, using sophisticated models, will be required for analysis of most supernova-remnant X-ray data in the future.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Dyer, KK and Reynolds, SP and Borkowski, KJ and Allen, GE and Petre, R}, year={2001}, month={Apr}, pages={439–453} }
 @article{borkowski_lyerly_reynolds_2001, title={Supernova remnants in the Sedov expansion phase: Thermal X-ray emission}, volume={548}, ISSN={["0004-637X"]}, DOI={10.1086/319011}, abstractNote={Improved calculations of X-ray spectra for supernova remnants (SNRs) in the Sedov-Taylor phase are reported, which for the first time include reliable atomic data for Fe L-shell lines. This new set of Sedov models also allows for a partial collisionless heating of electrons at the blast wave and for energy transfer from ions to electrons through Coulomb collisions. X-ray emission calculations are based on the updated Hamilton-Sarazin spectral model. The calculated X-ray spectra are successfully interpreted in terms of three distribution functions: the electron temperature and ionization timescale distributions, and the ionization timescale-averaged electron temperature distribution. The comparison of Sedov models with a frequently used single nonequilibrium ionization (NEI) timescale model reveals that this simple model is generally not an appropriate approximation to X-ray spectra of SNRs. We find instead that plane-parallel shocks provide a useful approximation to X-ray spectra of SNRs, particularly for young SNRs. Sedov X-ray models described here, together with simpler plane shock and single-ionization timescale models, have been implemented as standard models in the widely used XSPEC v11 spectral software package.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Borkowski, KJ and Lyerly, WJ and Reynolds, SP}, year={2001}, month={Feb}, pages={820–835} }
 @misc{reynolds_2001, title={Synchrotron radiation from galactic sources: What we can learn about particle acceleration}, volume={99}, ISSN={["0038-6308"]}, DOI={10.1023/A:1013892931294}, number={1-4}, journal={SPACE SCIENCE REVIEWS}, author={Reynolds, SP}, year={2001}, month={Oct}, pages={177–186} }
 @misc{drury_ellison_aharonian_berezhko_bykov_decourchelle_diehl_meynet_parizot_raymond_et al._2001, title={Test of Galactic Cosmic-Ray source models - Working group report}, volume={99}, ISSN={["1572-9672"]}, DOI={10.1023/A:1013825905795}, number={1-4}, journal={SPACE SCIENCE REVIEWS}, author={Drury, LO and Ellison, DE and Aharonian, FA and Berezhko, E and Bykov, A and Decourchelle, A and Diehl, R and Meynet, G and Parizot, E and Raymond, J and et al.}, year={2001}, month={Oct}, pages={329–352} }
 @article{borkowski_rho_reynolds_dyer_2001, title={Thermal and nonthermal X-ray emission in supernova remnant RCW 86}, volume={550}, ISSN={["0004-637X"]}, DOI={10.1086/319716}, abstractNote={Supernova remnants may exhibit both thermal and nonthermal X-ray emission. Such remnants can be distinguished by the weakness of their X-ray lines because of the presence of a strong nonthermal X-ray continuum. RCW 86 is a remnant with weak lines, resulting in low and peculiar abundances when thermal models alone are used to interpret its X-ray spectrum. This indicates the presence of a strong nonthermal synchrotron continuum. We analyze ASCA X-ray spectra of RCW 86 with the help of both nonequilibrium ionization thermal models and nonthermal synchrotron models. A two-temperature thermal model and a simple nonthermal model with an exponential cutoff (plus interstellar absorption) give reasonable results. We obtain a blast-wave velocity of 800 km s-1, a shock ionization age of 1 × 1011-3 × 1011 cm-3 s, and the break in nonthermal spectra at 2 × 1016-4 × 1016 Hz. The strength of the nonthermal continuum correlates well with the radio brightness in the bright southwest section of the remnant. This is convincing evidence for X-ray synchrotron emission in RCW 86.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Borkowski, KJ and Rho, J and Reynolds, SP and Dyer, KK}, year={2001}, month={Mar}, pages={334–345} }
 @article{safi-harb_petre_arnaud_keohane_borkowski_dyer_reynolds_hughes_2000, title={A broadband X-ray study of supernova remnant 3C 397}, volume={545}, ISSN={["1538-4357"]}, DOI={10.1086/317823}, abstractNote={We present a broadband imaging and spectral study of the radio-bright supernova remnant (SNR) 3C 397 with ROSAT, ASCA, and RXTE. A bright X-ray spot seen in the HRI image hints at the presence of a pulsar-powered component and gives this SNR a composite X-ray morphology. Combined ROSAT and ASCA imaging shows that the remnant is highly asymmetric, with its X-ray emission peaking at the western lobe. The hard-band images obtained with the ASCA Gas Imaging Spectrometer show that much of the hard X-ray emission arises from the western lobe, associated with the SNR shell, with little hard X-ray emission associated with the central hot spot. The spectrum from 3C 397 is heavily absorbed and dominated by thermal emission with emission lines evident from Mg, Si, S, Ar and Fe. Single-component models fail to describe the X-ray spectrum, and at least two components are required: a soft component characterized by a low temperature and a large ionization timescale, and a hard component required to account for the Fe-K emission line and characterized by a much lower ionization timescale. We use a set of nonequilibrium ionization (NEI) models (Borkowski et al., in preparation), and find that the fitted parameters are robust. The temperatures from the soft and hard components are ~0.2 keV and ~1.6 keV respectively. The corresponding ionization timescales n0t (n0 being the preshock hydrogen density) are ~6 × 1012 cm-3 s and ~6 × 1010 cm-3 s, respectively. The large n0t of the soft component suggests it is approaching ionization equilibrium; thus it can be fit equally well with a collisional equilibrium ionization model. The spectrum obtained with the Proportional Counter Array (PCA) of RXTE is contaminated by emission from the Galactic ridge, with only ~15% of the count rate originating from 3C 397 in the 5-15 keV range. The PCA spectrum allowed us to confirm the thermal nature of the hard X-ray emission. A third component originating from a pulsar-driven component is possible, but the contamination of the source signal by the Galactic ridge did not allow us to determine its parameters or find pulsations from any hidden pulsar. We discuss the X-ray spectrum in the light of two scenarios: a young ejecta-dominated remnant of a core-collapse SN, and a middle-aged SNR expanding in a dense ISM. In the first scenario, the hot component arises from the SNR shell, and the soft component from an ejecta-dominated component. 3C 397 would be a young SNR (a few thousand years old), but intermediate in dynamical age between the young historical shells (like Tycho or Kepler), and those that are well into the Sedov phase of evolution (like Vela). In the second scenario, the soft component represents the blast wave propagating in a dense medium, and the hard component is associated with hot gas encountering a fast shock, or arising from thermal conduction. In this latter scenario, the SNR would be ~twice as old, and transitioning into the radiative phase. The current picture we present in this paper is marginally consistent with this scenario, but it cannot be excluded. A spatially resolved spectroscopic study is needed to resolve the soft and hard components and differentiate between the two scenarios. Future Chandra and XMM data will also address the nature of the mysterious central (radio-quiet) X-ray spot.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Safi-Harb, S and Petre, R and Arnaud, KA and Keohane, JW and Borkowski, KJ and Dyer, KK and Reynolds, SP and Hughes, JP}, year={2000}, month={Dec}, pages={922–938} }
 @article{landecker_routledge_reynolds_smegal_borkowski_seward_1999, title={DA 530: A supernova remnant in a stellar wind bubble}, volume={527}, ISSN={["0004-637X"]}, DOI={10.1086/308100}, abstractNote={The high-latitude supernova remnant (SNR) DA 530 (G93.3+6.9), apparently a typical shell remnant, has highly polarized radio continuum emission and a very uniform circumferential magnetic field. We present new radio continuum (408 and 1420 MHz) and H I line observations, made with the Dominion Radio Astrophysical Observatory Synthesis Telescope, and we have made the first detection of X-ray emission from the SNR, using the ROSAT Position-Sensitive Proportional Counter. The SNR lies within a shell of H I, possibly created by an earlier stellar wind, whose kinematic distance is nominally 2.5 kpc but whose actual distance may be larger. The X-ray emission is extremely faint. A Raymond-Smith ionization-equilibrium model fits the data and suggests a very low density, ~0.05 cm-3, consistent with the occurrence of the supernova in a stellar wind cavity, but this model yields an explosion energy 100 times lower than the accepted value. A nonequilibrium shock model, incorporating a range of ionization timescales, is able to give more realistic physical parameters for the supernova remnant. On the balance of the evidence, we place DA 530 at a distance of 3.5 kpc, the largest distance permitted by the H I observations, where it lies 420 pc above the Galactic plane. The explosion, probably a Type Ia supernova, in a low-density cavity has resulted in weak X-ray emission and slow evolution. The explosion energy was 3.9 × 1050 ergs and the age is ~5000 years. The remnant, having swept up 3.9 M☉ in an ambient density of ~0.01 cm-3, is only now in the adiabatic phase, and this explains the absence of detected optical emission. Despite the low ambient density the efficiency of generation of synchrotron radio emission is ~0.4%, higher than in some historical SNRs. The ratio of radio to X-ray flux is about 100 times that for the remnant of SN 1006, which has comparable radio continuum properties. The very uniform magnetic field is not explained. DA 530 joins a small group of remnants at high Galactic latitude with unusual features, perhaps resulting from low ambient densities. Inhomogeneous nonequilibrium ionization models may be required for the interpretation of the X-ray emission from many other older SNRs.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Landecker, TL and Routledge, D and Reynolds, SP and Smegal, RJ and Borkowski, KJ and Seward, FD}, year={1999}, month={Dec}, pages={866–878} }
 @article{reynolds_keohane_1999, title={Maximum energies of shock-accelerated electrons in young shell supernova remnants}, volume={525}, ISSN={["0004-637X"]}, DOI={10.1086/307880}, abstractNote={Young supernova remnants (SNRs) are often assumed to be the source of cosmic rays up to energies approaching the slight steepening in the cosmic-ray spectrum at around 1000 TeV, known as the "knee." We show that the observed X-ray emission of 14 radio-bright shell remnants, including all five historical shells, can be used to put limits on Emax, the energy at which the electron energy distribution must steepen from its slope at radio-emitting energies. Most of the remnants show thermal spectra, so any synchrotron component must fall below the observed X-ray fluxes. We obtain upper limits on Emax by considering the most rapid physically plausible cutoff in the relativistic electron distribution, an exponential, which is as sharp or sharper than found in any more elaborate models. This maximally curved model then gives us the highest possible Emax consistent with not exceeding observed X-rays. Our results are thus independent of particular models for the electron spectrum in SNRs. Assuming homogeneous emitting volumes with a constant magnetic field strength of 10 μG, no object could reach 1000 TeV, and only one, Kes 73, has an upper limit on Emax above 100 TeV. All the other remnants have limits at or below 80 TeV. Emax is probably set by the finite remnant lifetime rather than by synchrotron losses for remnants younger than a few thousand years, so that an observed electron steepening should be accompanied by steepening at the same energy for protons. More complicated, inhomogeneous models could allow higher values of Emax in parts of the remnant, but the emission-weighted average value, that characteristic of typical electrons, should obey these limits. The young remnants are not expected to improve much over their remaining lives at producing the highest energy Galactic cosmic rays; if they cannot, this picture of cosmic-ray origin may need major alteration.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Reynolds, SP and Keohane, JW}, year={1999}, month={Nov}, pages={368–374} }
 @article{dyer_reynolds_1999, title={Multifrequency studies of bright radio supernova remnants. III. X-ray and radio}, volume={526}, ISSN={["0004-637X"]}, DOI={10.1086/307985}, abstractNote={Radio-bright, presumably young supernova remnants offer the opportunity of studying strong-shock physics and the nature of the interaction of ejected material with the surrounding medium. The relation between radio and X-ray morphology varies considerably among supernova remnants, with important implications for the physics of the emission processes at different wavelengths. We use Very Large Array (VLA) and Roentgen Satellite (ROSAT) images of the radio-bright supernova remnant 3C 397 (G41.1-0.3) to examine the shock structure in both thermal X-ray emission and nonthermal radio emission. The unusual rectangular morphology can be seen in VLA maps at 20 and 6 cm wavelength at a resolution of 6'' and in ROSAT HRI images. The X-ray images resemble the radio strongly, except for a small, possibly unresolved X-ray hot spot near the center. There is no variation in the X-ray hardness ratio from ROSAT Position Sensitive Proportional Counter data across the remnant, suggesting that at least between 0.4 and 2 keV the interior emission is not different in character from that in the bright shell regions. The remnant is unpolarized at 20 cm and has a mean fractional polarization of 1.5% ± 0.1% at 6 cm. The polarized flux, and polarized fraction, peak inside the remnant at a location not coincident with either an internal maximum in total-intensity radio emission or with the X-ray hot spot. Spectral index maps between 6 and 20 cm do not show any systematic differences associated with interior emission; there appears to be no "plerionic" or pulsar-driven component in 3C 397, at least as normally characterized by high polarization and a flat radio spectrum. Spectral index values spread about the mean by about Δα ~ 0.2, a result consistent with previous work. The steep total-intensity profile off the southwest edge of 3C 397 allows an inference of the upstream electron diffusion coefficient and implies a mean free path for electron scattering shorter than in the general interstellar medium but longer than that similarly inferred for Tycho and SN 1006. A simple analysis based on the observed X-ray flux gives an estimate of the mean density in 3C 397 of about 4 cm-3, which would also be enough to depolarize the 20 cm emission completely, as observed. The remnant age is then of order 103 yr, and the current shock velocity is about 1600 km s-1. Finally, we speculate on possible mechanisms producing the X-ray hot spot.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Dyer, KK and Reynolds, SP}, year={1999}, month={Nov}, pages={365–384} }
 @article{reynolds_1999, title={Predicted images and spectra of gamma rays and x-rays from shell supernova remnants}, volume={38}, number={1-6}, journal={Astrophysical Letters & Communications}, author={Reynolds, S. P.}, year={1999}, pages={425} }
 @article{baring_ellison_reynolds_grenier_goret_1999, title={Radio to gamma-ray emission from shell-type supernova remnants: Predictions from nonlinear shock acceleration models}, volume={513}, ISSN={["1538-4357"]}, DOI={10.1086/306829}, abstractNote={Supernova remnants (SNRs) are widely believed to be the principal source of Galactic cosmic rays, produced by diffusive shock acceleration in the environs of the remnant's expanding blast wave. Such energetic particles can produce gamma rays and lower energy photons via interactions with the ambient plasma. The recently reported observation of TeV gamma rays from SN 1006 by the Collaboration of Australia and Nippon for a Gamma-Ray Observatory in the Outback (CANGAROO), combined with the fact that several unidentified EGRET sources have been associated with known radio/optical/X-ray-emitting remnants, provides powerful motivation for studying gamma-ray emission from SNRs. In this paper, we present results from a Monte Carlo simulation of nonlinear shock structure and acceleration coupled with photon emission in shelllike SNRs. These nonlinearities are a by-product of the dynamical influence of the accelerated cosmic rays on the shocked plasma and result in distributions of cosmic rays that deviate from pure power laws. Such deviations are crucial to acceleration efficiency considerations and impact photon intensities and spectral shapes at all energies, producing GeV/TeV intensity ratios that are quite different from test particle predictions. The Sedov scaling solution for SNR expansions is used to estimate important shock parameters for input into the Monte Carlo simulation. We calculate ion (proton and helium) and electron distributions that spawn neutral pion decay, bremsstrahlung, inverse Compton, and synchrotron emission, yielding complete photon spectra from radio frequencies to gamma-ray energies. The cessation of acceleration caused by the spatial and temporal limitations of the expanding SNR shell in moderately dense interstellar regions can yield spectral cutoffs in the TeV energy range that are consistent with Whipple's TeV upper limits on those EGRET unidentified sources that have SNR associations. Supernova remnants in lower density environments generate higher energy cosmic rays that produce predominantly inverse Compton emission observable at super-TeV energies, consistent with the SN 1006 detection. In general, sources in such low-density regions will be gamma-ray-dim at GeV energies.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Baring, MG and Ellison, DC and Reynolds, SP and Grenier, IA and Goret, P}, year={1999}, month={Mar}, pages={311–338} }
 @article{wilner_reynolds_moffett_1998, title={CO observations toward the supernova remnant 3C 391}, volume={115}, ISSN={["0004-6256"]}, DOI={10.1086/300190}, abstractNote={We present observations of CO J = 1–0 emission toward 3C 391, a supernova remnant whose radio continuum and X-ray emission suggest evolution near a strong density gradient in the surrounding medium. The CO maps reveal that the remnant is located at the edge of a molecular cloud, and the CO emission shows a striking correspondence with the "breakout" morphology of the remnant traced by radio continuum emission. These data support the idea that the progenitor star exploded within a dense molecular cloud and that the supernova blast wave has now broken out through the cloud boundary. Bright radio emission coincident with strong CO emission suggests that the blast wave accelerates electrons to energies of at least 20 GeV, even as it moves into dense neutral material.}, number={1}, journal={ASTRONOMICAL JOURNAL}, author={Wilner, DJ and Reynolds, SP and Moffett, DA}, year={1998}, month={Jan}, pages={247–251} }
 @article{yusef-zadeh_cotton_reynolds_1998, title={G359.87+0.18: A young supernova remnant candidate near the Galactic center?}, volume={498}, ISSN={["0004-637X"]}, DOI={10.1086/311294}, abstractNote={Subarcsecond radio continuum observations of the Galactic center region at λ = 6 and 2 cm reveal a 0.″5 diameter source with a shell-like morphology. This source is linearly polarized at a level of 16% at 6 cm and has a steep nonthermal spectrum with a spectral index of 1.6 between 6 and 2 cm. The distance to this source is not known, but the large rotation measure value of 3000 rad m−2 suggests that G359.87+0.18 is likely to be located in the inner Galaxy or at an extragalactic distance. We discuss possible interpretations of this object as a recent supernova, a very young supernova remnant, a nova remnant, or an extragalactic source. All possibilities are highly problematic.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Yusef-Zadeh, F and Cotton, WD and Reynolds, SP}, year={1998}, month={May}, pages={L55–L58} }
 @article{reynolds_1998, title={Models of synchrotron X-rays from shell supernova remnants}, volume={493}, ISSN={["0004-637X"]}, DOI={10.1086/305103}, abstractNote={The diffusive shock acceleration process can accelerate particles to a maximum energy depending on the shock speed and age and on any competing loss processes on the particles. The shock waves of young supernova remnants can easily accelerate electrons to energies in excess of 1 TeV, where they can produce X-rays by the synchrotron process. I describe a detailed calculation of the morphology and spectrum of synchrotron X-rays from supernova remnants. Remnants are assumed to be spherical and in the Sedov evolutionary phase, though the results are insensitive to the detailed dynamics. The upstream magnetic field is assumed uniform; downstream it is assumed to be compressed but not additionally turbulently amplified. In all cases, spectra begin to depart from power laws somewhere in the optical to UV range and roll off smoothly through the X-ray band. I show that simple approximations for the electron emissivity are not adequate; a full convolution of the individual electron synchrotron emissivity with a calculated electron distribution at each point in the remnant is required. Models limited by the finite shock age, by synchrotron or inverse Compton losses on electrons, or by escape of electrons above some energy have characteristically different spectral shapes, but within each class, models resemble one another strongly and can be related by simple scalings. The images and spectra depend primarily on the remnant age, the upstream magnetic field strength, and the level of magnetic turbulence near the shock in which the electrons scatter. In addition, images depend on the viewing or aspect angle between the upstream magnetic field and the line of sight. The diffusion coefficient is assumed to be proportional to particle energy (or mean free path proportional to gyroradius), but I investigate the possibility that the proportionality constant becomes much larger above some energy, corresponding to an absence of long-wavelength MHD waves. Models producing similar spectra may differ significantly in morphology, which allows for possible discriminations. I parameterize the model spectra in terms of a slope at 4 keV and a factor by which the X-ray flux density at that energy falls below the extrapolated radio spectrum. Synchrotron radiation may contribute significantly to the X-ray emission of remnants up to several thousand years old.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Reynolds, SP}, year={1998}, month={Jan}, pages={375–396} }
 @article{blondin_wright_borkowski_reynolds_1998, title={Transition to the radiative phase in supernova remnants}, volume={500}, ISSN={["0004-637X"]}, DOI={10.1086/305708}, abstractNote={The evolution of a supernova remnant through the transition from an adiabatic Sedov-Taylor blast wave to a radiative pressure-driven snowplow phase is studied using one- and two-dimensional hydrodynamic simulations. This transition is marked by a catastrophic collapse of the postshock gas, forming a thin, dense shell behind the forward shock. After the transition, the shock front is characterized by a deceleration parameter, Vt/R ≈ 0.33, which is considerably higher than the analytic estimate of for a pressure-driven snowplow. In two dimensions, the catastrophic collapse is accompanied by violent dynamical instabilities of the thin, cool shell. The violence of the collapse and the subsequent instability of the shell increase with increasing ambient density. Preshock density perturbations as small as 1% in an ambient medium with density of 100 cm-3 can lead to distortions of the shock front larger than 10% of the radius of the remnant.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Blondin, JM and Wright, EB and Borkowski, KJ and Reynolds, SP}, year={1998}, month={Jun}, pages={342–354} }
 @article{reynoso_moffett_goss_dubner_dickel_reynolds_giacani_1997, title={A VLA study of the expansion of Tycho's supernova remnant}, volume={491}, ISSN={["0004-637X"]}, DOI={10.1086/304997}, abstractNote={We have measured the expansion of Tycho's supernova remnant (SNR) over a 10 yr interval by comparing new VLA observations at 1375 MHz made in 1994 and 1995 with previous observations made in 1983 and 1984 using the same array configurations, bandwidths, calibrators, and integration times. To compute the expansion of the outer rim, we estimate the expansion rate for radial sectors of 4°. The overall mean expansion obtained is 3.″05 but varies between 1'' and 5'' around the shell. The weighted average fractional expansion rate is 0.1126% yr-1, corresponding to a power-law index (expansion parameter) ν ≡ d ln R/d ln t = 0.471, with an error of ~6%. We also compute the expansion of interior features, and find a comparable global expansion parameter. The value of ν obtained is in excess of the pure Sedov value of 0.4, indicating that Tycho's SNR is still in transition from an earlier phase to the Sedov adiabatic phase of evolution, and supporting expansion into a medium without a strong radial density gradient, as expected for a Type Ia supernova. In addition, the significant local variations that we observe suggest that a unique average expansion rate is an oversimplified description for Tycho. Both radio flux and polarization were found to remain almost constant during the intervening 10 years.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Reynoso, EM and Moffett, DA and Goss, WM and Dubner, GM and Dickel, JR and Reynolds, SP and Giacani, EB}, year={1997}, month={Dec}, pages={816–828} }