@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{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{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{guest_blair_borkowski_ghavamian_hendrick_long_petre_raymond_rest_sankrit_et al._2022, title={Locating the CSM Emission within the Type Ia Supernova Remnant N103B}, volume={926}, ISSN={["1538-4357"]}, DOI={10.3847/1538-4357/ac4913}, abstractNote={We present results from deep Chandra observations of the young Type Ia supernova remnant (SNR) 0509–68.7, also known as N103B, located in the Large Magellanic Cloud (LMC). The remnant displays an asymmetry in brightness, with the western hemisphere appearing significantly brighter than the eastern one. Previous multiwavelength observations have attributed the difference to a density gradient and suggested origins in circumstellar material, drawing similarities to Kepler’s SNR. We apply a clustering technique combined with traditional imaging analysis to spatially locate various emission components within the remnant. We find that O and Mg emission is strongest along the blast wave, and coincides with Spitzer observations of dust emission and optical emission from the nonradiative shocks. The abundances of O and Mg in these regions are enhanced relative to the average LMC abundances and appear as a distinct spatial distribution compared to the ejecta products, supporting the interpretation based on a circumstellar medium. We also find that the spatial distribution of Cr is identical to that of Fe in the interior of the remnant, and does not coincide at all with the O and Mg emission.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Guest, Benson T. 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 Sankrit, Ravi and et al.}, year={2022}, month={Feb} }
 @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_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}, number={1}, journal={Astrophysical Journal Letters}, author={Borkowski, K. J. and Gwynne, P. and Reynolds, S. P. and Green, D. A. and Hwang, U. and Petre, R. and Willett, R.}, year={2017} }
 @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{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{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{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{sankrit_raymond_bautista_gaetz_williams_blair_borkowski_long_2014, title={SPITZER IRS OBSERVATIONS OF THE XA REGION IN THE CYGNUS LOOP SUPERNOVA REMNANT}, volume={787}, ISSN={["1538-4357"]}, DOI={10.1088/0004-637x/787/1/3}, abstractNote={We report on spectra of two positions in the XA region of the Cygnus Loop supernova remnant obtained with the InfraRed Spectrograph on the Spitzer Space Telescope. The spectra span the 10–35 μm wavelength range, which contains a number of collisionally excited forbidden lines. These data are supplemented by optical spectra obtained at the Whipple Observatory and an archival UV spectrum from the International Ultraviolet Explorer. Coverage from the UV through the IR provides tests of shock wave models and tight constraints on model parameters. Only lines from high ionization species are detected in the spectrum of a filament on the edge of the remnant. The filament traces a 180 km s−1 shock that has just begun to cool, and the oxygen to neon abundance ratio lies in the normal range found for Galactic H ii regions. Lines from both high and low ionization species are detected in the spectrum of the cusp of a shock–cloud interaction, which lies within the remnant boundary. The spectrum of the cusp region is matched by a shock of about 150 km s−1 that has cooled and begun to recombine. The post-shock region has a swept-up column density of about 1.3 × 1018 cm−2, and the gas has reached a temperature of 7000–8000 K. The spectrum of the Cusp indicates that roughly half of the refractory silicon and iron atoms have been liberated from the grains. Dust emission is not detected at either position.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Sankrit, Ravi and Raymond, John C. and Bautista, Manuel and Gaetz, Terrance J. and Williams, Brian J. and Blair, William P. and Borkowski, Kazimierz J. and Long, Knox S.}, year={2014}, month={May} }
 @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{raymond_ghavamian_williams_blair_borkowski_gaetz_sankrit_2013, title={Grain destruction in a supernova remnant shock wave}, volume={778}, DOI={10.1088/0004-637x/778/2/161}, abstractNote={Dust grains are sputtered away in the hot gas behind shock fronts in supernova remnants (SNRs), gradually enriching the gas phase with refractory elements. We have measured emission in C iv λ1550 from C atoms sputtered from dust in the gas behind a non-radiative shock wave in the northern Cygnus Loop. Overall, the intensity observed behind the shock agrees approximately with predictions from model calculations that match the Spitzer 24 μm and the X-ray intensity profiles. Thus, these observations confirm the overall picture of dust destruction in SNR shocks and the sputtering rates used in models. However, there is a discrepancy in that the C iv intensity 10″ behind the shock is too high compared with the intensities at the shock and 25″ behind it. Variations in the density, hydrogen neutral fraction, and the dust properties over parsec scales in the pre-shock medium limit our ability to test dust destruction models in detail.}, number={2}, journal={Astrophysical Journal}, author={Raymond, J. C. and Ghavamian, P. and Williams, B. J. and Blair, W. P. and Borkowski, K. J. and Gaetz, T. J. and Sankrit, R.}, year={2013} }
 @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{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{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{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{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{pannuti_rho_borkowski_cameron_2010, title={MIXED-MORPHOLOGY SUPERNOVA REMNANTS IN X-RAYS: ISOTHERMAL PLASMA IN HB21 AND PROBABLE OXYGEN-RICH EJECTA IN CTB 1}, volume={140}, ISSN={["0004-6256"]}, DOI={10.1088/0004-6256/140/6/1787}, abstractNote={We present an analysis of X-ray observations of the Galactic supernova remnants (SNRs) HB21 (G89.0+4.7) and CTB 1 (G116.9+0.2), two well-known members of the class of mixed-morphology (MM) SNRs. Our analysis draws upon observations of both SNRs made with the Advanced Satellite for Cosmology and Astrophysics (ASCA): we have also used an archived Chandra observation of CTB 1 as part of this effort. We find a marked contrast between the X-ray properties of HB21 and CTB 1: in the case of HB21, the extracted spectra of the northwest and southeast regions of the X-ray-emitting plasma associated with the SNR can be fit with a single thermal model with marginally enhanced silicon and sulfur abundances. For both of these regions, the derived column density and temperature are NH ∼ 0.3 × 1022 cm−2 and kT ∼ 0.7 keV, respectively. No significant spatial differences in temperature or elemental abundances between the two regions are detected and the X-ray-emitting plasma for both regions is close to ionization equilibrium. Our Chandra spectral analysis of CTB 1 reveals that this source is likely an oxygen-rich SNR with enhanced abundances of oxygen and neon: this result is quite surprising for an evolved SNR like CTB 1. The high angular resolution Chandra observation of CTB 1 reveals spectral variations across this SNR: in particular, we have detected localized hard emission with an angular extent of ∼1′. The extracted ASCA spectra for both the southwest and northeastern regions of CTB 1 cannot be fit with a single thermal component and instead an additional component is required to account for the presence of excess emission seen at higher energies. Based on our fits to the extracted ASCA spectra, we derive a column density NH ∼ 0.6 × 1022 cm−2 and a temperature for the soft thermal component of kTsoft ∼ 0.28 keV for both regions. The hard emission from the southwest region may be modeled with either a thermal component with a temperature kThard ∼ 3 keV or by a power-law component with a photon index Γ ∼ 2–3; for the northeast region, the hard emission may be modeled with a power-law component with a photon index Γ = 1.4. The detection of center-filled ejecta-dominated X-ray emission from HB21 and CTB 1 as well as other MM SNRs suggests a new scenario for the origin of the contrasting X-ray and radio morphologies of this class of sources. Lastly, we have analyzed the properties of the discrete hard X-ray source 1WGA J0001.4+6229 which is seen in projection just inside the northeastern shell of CTB 1. Our extracted ASCA GIS spectra of this source are best fit using a power-law model with a photon index Γ = 2.2+0.5−1.2: this slope is typical for featureless power-law continua produced by rotation-powered pulsars. This source may be a neutron star associated with CTB 1. We find marginal evidence for X-ray pulsations from this source with a period of 47.6154 ms. A deep radio observation of this source failed to reveal any pulsations.}, number={6}, journal={ASTRONOMICAL JOURNAL}, author={Pannuti, Thomas G. and Rho, Jeonghee and Borkowski, Kazimierz J. and Cameron, P. Brian}, year={2010}, month={Dec}, pages={1787–1805} }
 @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{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{zhekov_mccray_dewey_canizares_borkowski_burrows_park_2009, title={HIGH-RESOLUTION X-RAY SPECTROSCOPY OF SNR 1987A: CHANDRA LETG AND HETG OBSERVATIONS IN 2007}, volume={692}, ISSN={0004-637X 1538-4357}, url={http://dx.doi.org/10.1088/0004-637X/692/2/1190}, DOI={10.1088/0004-637X/692/2/1190}, abstractNote={We present an extended analysis of the deep Chandra LETG and HETG observations of the supernova remnant 1987A (SNR 1987A) carried out in 2007. The global fits to the grating spectra show that the temperature of the X-ray emitting plasma in the slower shocks in this system has remained stable for the last three years, while that in the faster shocks has decreased. This temperature evolution is confirmed by the first light curves of strong X-ray emission lines and their ratios. On the other hand, bulk gas velocities inferred from the X-ray line profiles are too low to account for the postshock plasma temperatures inferred from spectral fits. This suggests that the X-ray emission comes from gas that has been shocked twice, first by the blast wave and again by shocks reflected from the inner ring of SNR 1987A. A new model that takes these considerations into account gives support to this physical picture.}, number={2}, journal={The Astrophysical Journal}, publisher={IOP Publishing}, author={Zhekov, Svetozar A. and McCray, Richard and Dewey, Daniel and Canizares, Claude R. and Borkowski, Kazimierz J. and Burrows, David N. and Park, Sangwook}, year={2009}, month={Feb}, pages={1190–1204} }
 @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_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{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} }
 @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{zhekov_mccray_borkowski_burrows_park_2006, title={Chandra LETG observations of supernova remnant 1987A}, volume={645}, ISSN={["1538-4357"]}, DOI={10.1086/504285}, abstractNote={We discuss the results from deep Chandra LETG observations of the supernova remnant 1987A (SNR 1987A). We find that a distribution of shocks, spanning the same range of velocities (from ~300 to 1700 km s-1) as deduced in the first part of our analysis, can account for the entire X-ray spectrum of this object. The postshock temperature distribution is bimodal, peaking at kT ~ 0.5 and ~3 keV. Abundances inferred from the X-ray spectrum have values similar to those for the inner circumstellar ring, except that the abundances of nitrogen and oxygen are approximately a factor of 2 lower than those inferred from the optical/UV spectrum. The velocity of the X-ray-emitting plasma has decreased since 1999, apparently because the blast wave has entered the main body of the inner circumstellar ring.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Zhekov, Svetozar A. and McCray, Richard and Borkowski, Kazimierz J. and Burrows, David N. and Park, Sangwook}, year={2006}, month={Jul}, pages={293–302} }
 @article{badenes_borkowski_hughes_hwang_bravo_2006, title={Constraints on the physics of type Ia supernovae from the X-ray spectrum of the Tycho supernova remnant}, volume={645}, ISSN={["1538-4357"]}, DOI={10.1086/504399}, abstractNote={In this paper we use high-quality X-ray observations from XMM-Newton and Chandra to gain new insights into the explosion that originated Tycho's supernova 433 yr ago. We perform a detailed comparison between the ejecta emission from the spatially integrated X-ray spectrum of the supernova remnant and current models for Type Ia supernova explosions. We use a grid of synthetic X-ray spectra based on hydrodynamic models of the evolution of the supernova remnant and nonequilibrium ionization calculations for the state of the shocked plasma. We find that the fundamental properties of the X-ray emission in Tycho are well reproduced by a one-dimensional delayed detonation model with a kinetic energy of ~1.2 × 1051 ergs. All the other paradigms for Type Ia explosions that we have tested, including one-dimensional deflagrations, pulsating delayed detonations, and sub-Chandrasekhar explosions, as well as deflagration models calculated in three dimensions, fail to provide a good approximation of the observed ejecta emission. Our results require that the supernova ejecta retain some degree of chemical stratification, with Fe peak elements interior to intermediate-mass elements. This strongly suggests that a supersonic burning front (i.e., a detonation) must be involved at some stage in the physics of Type Ia supernova explosions.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Badenes, Carles and Borkowski, Kazimierz J. and Hughes, John P. and Hwang, Una and Bravo, Eduardo}, year={2006}, month={Jul}, pages={1373–1391} }
 @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{fesen_hammell_morse_chevalier_borkowski_dopita_gerardy_lawrence_raymond_bergh_2006, title={Discovery of outlying high-velocity oxygen-rich ejecta in Cassiopeia A}, volume={636}, ISSN={["0004-637X"]}, DOI={10.1086/498092}, abstractNote={Analysis of broadband HST ACS and WFPC2 images of the young Galactic supernova remnant Cassiopeia A reveals a far larger population of outlying, high-velocity knots of ejecta with a broader range of chemical properties than previously suspected. In this paper, we concentrate on a ≃1.5 arcmin2 region located along the eastern limb of the remnant where we identify three main classes of outer ejecta: (1) knots dominated by [N II] λλ6548, 6583 emission, (2) knots dominated by oxygen emission lines, especially [O II] λλ7319, 7330, and (3) knots with emission-line strengths similar to the [S II]-strong fast-moving knot (FMK) ejecta commonly seen in the main emission shell. Mean transverse velocities derived from observed proper motion for N-rich, O-rich, and FMK-like knots identified in this region were found to be 8100, 7900, and 7600 km s-1, respectively. The discovery of a significant population of O-rich ejecta situated between the suspected N-rich outer photospheric layer and S-rich FMK-like ejecta suggests that the progenitor's chemical layers were not completely disrupted by the supernova explosion outside of the remnant's northeast and southwest high-velocity "jet" regions. In addition, we find the majority of O-rich outer ejecta at projected locations out beyond the remnant's fastest moving Fe-rich X-ray emission material seen in Chandra and XMM-Newton data along the eastern limb, suggesting that penetration of Fe-rich material up through the S- and Si-rich mantle did not extend past the progenitor's N- or O-rich outer layers for this section of the remnant.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Fesen, RA and Hammell, MC and Morse, J and Chevalier, RA and Borkowski, KJ and Dopita, MA and Gerardy, CL and Lawrence, SS and Raymond, JC and Bergh, S}, year={2006}, month={Jan}, pages={859–872} }
 @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{fesen_hammell_morse_chevalier_borkowski_dopita_gerardy_lawrence_raymond_bergh_2006, title={The expansion asymmetry and age of the Cassiopeia A supernova remnant}, volume={645}, ISSN={["1538-4357"]}, DOI={10.1086/504254}, abstractNote={HST images of the young supernova remnant Cas A are used to explore the expansion and spatial distribution of its highest velocity debris. ACS WFC images taken in 2004 March and December with Sloan F625W, F775W, and F850LP filters were used to identify 1825 high-velocity, outlying ejecta knots through measured proper motions of 0.″35-0.″90 yr-1, corresponding to Vtrans = 5500-14,500 km s-1 assuming d = 3.4 kpc. The distribution of derived transverse expansion velocities for these ejecta knots shows a striking bipolar asymmetry with the highest velocity knots (Vtrans ≥ 10,500 km s-1) confined to nearly opposing northeast and southwest "jets" at P.A. = 45°-70° and 230°-270°, respectively. The jets have about the same maximum expansion velocity of ≃14,000 km s-1 and appear kinematically and chemically distinct in that they are the remnant's only S-rich ejecta with expansion velocities above the 10,000-11,000 km s-1 exhibited by outer nitrogen-rich ejecta, which otherwise represent the remnant's highest velocity debris. In addition, we find significant gaps in the spatial distribution of outlying ejecta in directions that are approximately perpendicular to the jets (P.A. = 145°-200° and 335°-350°). The remnant's central X-ray point source lies some 7'' to the southeast of the estimated expansion center (P.A. = 169° ± 8.°4) indicating a projected motion toward the middle of the broad southern ejecta knot gap. Extrapolations of measured 9 month proper motions for all 1825 outer ejecta knots and a selected subsample of 72 bright and compact knots suggest explosion dates (assuming no knot deceleration) of 1662 ± 27 and 1672 ± 18, respectively. We find some evidence for nonuniform deceleration in different directions around the remnant and find 126 knots located along the northwestern limb among the least decelerated ejecta, suggesting a convergence date of 1681 ± 19. A remnant age of around 325 yr would imply a ≃350 km s-1 transverse velocity for the central X-ray point source.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Fesen, Robert A. and Hammell, Molly C. and Morse, Jon and Chevalier, Roger A. and Borkowski, Kazimierz J. and Dopita, Michael A. and Gerardy, Christopher L. and Lawrence, Stephen S. and Raymond, John C. and Bergh, Sidney}, year={2006}, month={Jul}, pages={283–292} }
 @article{badenes_bravo_borkowski_2005, title={A model grid for the spectral analysis of X-ray emission in young Type Ia supernova remnants}, volume={35}, ISBN={["*************"]}, ISSN={["0273-1177"]}, DOI={10.1016/j.asr.2005.01.019}, abstractNote={We address a new set of models for the spectral analysis of the X-ray emission from young, ejecta-dominated Type Ia supernova remnants. These models are based on hydrodynamic simulations of the interaction between Type Ia supernova explosion models and the surrounding ambient medium, coupled to self-consistent ionization and electron heating calculations in the shocked supernova ejecta, and the generation of synthetic spectra with an appropriate spectral code. The details are provided elsewhere, but in this paper we concentrate on a specific class of Type Ia explosion models (delayed detonations), commenting on the differences that arise between their synthetic X-ray spectra under a variety of conditions.}, number={6}, journal={YOUNG NEUTRON STARS AND SUPERNOVA REMNANTS}, author={Badenes, C and Bravo, E and Borkowski, KJ}, year={2005}, pages={987–990} }
 @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{zhekov_mccray_borkowski_burrows_park_2005, title={Chandra observations of shock kinematics in supernova remnant 1987A}, volume={628}, ISSN={["1538-4357"]}, DOI={10.1086/432794}, abstractNote={We report the first results from deep X-ray observations of the supernova remnant SNR 1987A with the Chandra LETG. Temperatures inferred from line ratios range from ~0.1 to 2 keV and increase with ionization potential. Expansion velocities inferred from X-ray line profiles range from ~300 to 1700 km s-1, much less than the velocities inferred from the radial expansion of the radio and X-ray images. We can account for these observations with a scenario in which the X-rays are emitted by shocks produced where the supernova blast wave strikes dense protrusions of the inner circumstellar ring, which are also responsible for the optical hot spots.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Zhekov, SA and McCray, R and Borkowski, KJ and Burrows, DN and Park, S}, year={2005}, month={Aug}, pages={L127–L130} }
 @article{badenes_borkowski_bravo_2005, title={Thermal X-ray emission from shocked ejecta in Type Ia supernova remnants. II. Parameters affecting the spectrum}, volume={624}, ISSN={["1538-4357"]}, DOI={10.1086/428829}, abstractNote={The supernova remnants (SNRs) left behind by Type Ia supernovae (SNe) provide an excellent opportunity for the study of these enigmatic objects. In a previous work we showed that it is possible to use the X-ray spectra of young Type Ia SNRs to explore the physics of Type Ia SNe and identify the relevant mechanism underlying these explosions. Our simulation technique is based on hydrodynamic and nonequilibrium ionization calculations of the interaction of a grid of Type Ia explosion models with the surrounding ambient medium, coupled to an X-ray spectral code. In this work we explore the influence of two key parameters on the shape of the X-ray spectrum of the ejecta: the density of the ambient medium around the SN progenitor and the efficiency of collisionless electron heating at the reverse shock. We also discuss the performance of recent three-dimensional simulations of Type Ia SN explosions in the context of the X-ray spectra of young SNRs. We find a better agreement with the observations for Type Ia SN models with stratified ejecta than for three-dimensional deflagration models with well-mixed ejecta. We conclude that our grid of Type Ia SNR models can improve our understanding of these objects and their relationship to the SNe that originated them.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Badenes, C and Borkowski, KJ and Bravo, E}, year={2005}, month={May}, pages={198–212} }
 @article{morse_fesen_chevalier_borkowski_gerardy_lawrence_van den bergh_2004, title={Location of the optical reverse shock in the Cassiopeia A supernova remnant}, volume={614}, DOI={10.1086/423709}, abstractNote={We use two epochs of Hubble Space Telescope WFPC2 images separated by 2 yr to determine the location and propagation of the reverse shock (RS) in the young supernova remnant Cassiopeia A (Cas A). The images trace optical line emission from fast-moving knots and filaments of highly processed ejecta as they cross the RS, become heated and compressed, and radiatively cool. At numerous positions around the optical shell, new emission features are seen in the 2002 images that were not yet visible in the 2000 exposures. In a few instances emission features seen in the first epoch have completely disappeared in the second epoch. We concentrate on two regions along the rim of the main emission shell in Cas A for close inspection: one in the northwestern part of the shell and another along the southwestern part of it. In these regions the RS is viewed almost edge-on, and its precise position has been measured. The RS is coherent in these regions over arcminute (~1 pc) scales but is highly distorted perpendicular to the direction of expansion. We find the RS to be generally expanding at 50%-60% of the ~5500 km s-1 bulk velocity of the optical ejecta. We present shock models for the ejecta that are consistent with the high densities and short cooling times observed in the optical knots of the Cas A remnant.}, number={2}, journal={Astrophysical Journal}, author={Morse, J. A. and Fesen, R. A. and Chevalier, R. A. and Borkowski, K. J. and Gerardy, C. L. and Lawrence, S. S. and Van Den Bergh, S.}, year={2004}, pages={727–736} }
 @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{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} }
 @inproceedings{harrington_li_borkowski_2003, title={HST measurements of the angular expansion, kinematics and distance of the planetary nebula BD+30 degrees 3639}, DOI={10.1017/s007418090020939x}, abstractNote={The HST WFPC2 camera was used to obtain images of the planetary nebula BD+30°3639 at two epochs separated by 5.66 years. The expansion of the nebula in Hα and [N II] has been measured using several methods. Detailed expansion maps for both emission lines were constructed from nearly 200 almost independent features. There is good agreement between the (independent) Hα and [N II] proper motions. There are clear deviations from uniform radial expansion, with higher expansion rates in regions where the shell is faintest, such as the south-west quadrant. HST STIS echelle spectra obtained in the C II] λ2326 multiplet provide well-resolved expansion velocities at two position angles. We find that the central velocity split is ±36.3 km s -1 at a position angle of 99°. To determine the distance of BD+30°3639 by comparison of the angular expansion and the radial expansion, we must address the problem of the three dimensional shape of the nebula. We measured the angular expansion along the position of the 99° echelle slit, finding displacements of 4.25 mas yr -1 at the shell edge (2″.47 from the center). If the nebula were spherical, this would imply a distance of 1.80 kpc. But there is evidence that the nebula is elongated along the line of sight, which suggests that the actual distance is less. Radio continuum images from 5 and 15 GHz VLA observations provide information on the extent of the radial elongation. We fit the radio brightness variation and the echelle data by approximating the nebula as an ellipsoid. Our model has an axial ratio of 1.56, is inclined to the line of sight by 9°.7, and exhibits an expansion in the plane of the sky which is 2/3 that in the radial direction, leading to a distance of 1.2 kpc. Based on a recent model atmosphere (Crowther et al., these proceedings), this distance implies a stellar luminosity of 4250 L ⊙ .}, number={209}, booktitle={Planetary nebulae: Their evolution and role in the universe: Proceedings of the 209th symposium of the International Astronomical Union held at Canberra, Australia, 19-23 November 2001 (1st. ed.) (IAU Symposia ; 209).}, publisher={Published on behalf of IAU by the Astronomical Society of the Pacific}, author={Harrington, J. P. and Li, J. Y. and Borkowski, K. J.}, editor={S. Kwok, M. A. Dopita and Sutherland, R. S.Editors}, year={2003}, pages={480} }
 @article{badenes_bravo_borkowski_dominquez_2003, title={Thermal X-ray emission from shocked ejecta in type Ia supernova remnants: Prospects for explosion mechanism identification}, volume={593}, ISSN={["1538-4357"]}, DOI={10.1086/376448}, abstractNote={The explosion mechanism behind Type Ia supernovae is a matter of continuing debate. The diverse attempts to identify or at least constrain the physical processes involved in the explosion have been only partially successful so far. In this paper we propose to use the thermal X-ray emission from young supernova remnants (SNRs) originating in Type Ia events to extract relevant information concerning the explosions themselves. We have produced a grid of thermonuclear supernova models representative of the paradigms currently under debate: pure deflagrations, delayed detonations, pulsating delayed detonations, and sub-Chandrasekhar explosions, using their density and chemical composition profiles to simulate the interaction with the surrounding ambient medium and the ensuing plasma heating, nonequilibrium ionization and thermal X-ray emission of the ejecta. Key observational parameters such as electron temperatures, emission measures, and ionization timescales are presented and discussed. We find that not only is it possible to identify the explosion mechanism from the spectra of young Type Ia SNRs, it is in fact necessary to take the detailed ejecta structure into account if such spectra are to be modeled in a self-consistent way. Neither element line flux ratios nor element emission measures are good estimates of the true ratios of ejected masses, with differences of as much as 2 or 3 orders of magnitude for a given model. Comparison with observations of the Tycho SNR suggests a delayed detonation as the most probable explosion mechanism. Line strengths, line ratios, and the centroid of the Fe Kα line are reasonably well reproduced by a model of this kind.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Badenes, C and Bravo, E and Borkowski, KJ and Dominquez, I}, year={2003}, month={Aug}, pages={358–369} }
 @article{rho_borkowski_2002, title={ROSAT/ASCA observations of the mixed-morphology supernova remnant W28}, volume={575}, ISSN={["1538-4357"]}, DOI={10.1086/341192}, abstractNote={We present three sets of ROSAT Position Sensitive Proportional Counter and four sets of ASCA observations of the supernova remnant (SNR) W28. The overall shape of X-ray emission in W28 is elliptical, dominated by a centrally concentrated interior emission, sharply peaked at the center. There are also partial northeastern and southwestern shells, and both the central and shell X-ray emission is highly patchy. The ASCA spectra reveal emission lines of Ne, Mg, Si, and Fe Kα and continuum extending at least up to 7 keV, showing that X-ray emission in W28 is mostly of thermal origin with a hot thermal component. We found that spectral variations are present in W28. The southwestern shell can be fitted well by a plane-shock model with a temperature of 1.5 keV and an ionization timescale of 1.5 × 1011 cm-3 s. The long ionization timescale combined with a low estimated electron density of ~0.2 cm-3 imply an SNR age of several times 104 yr. The low density in the southwest is consistent with the shock breakout away from molecular clouds in the north and northeast. The northeastern shell, with a lower temperature of 0.56 keV and a longer ionization timescale of 1.7 × 1013 cm-3 s, spatially coincides with the radiative shell delineated by radio and optical filaments, but a relatively high temperature and a low density of X-ray-emitting gas in the northeastern shell indicate that we are not observing gas cooling from high temperatures. Unlike for the southwestern and northeastern shells, the central emission cannot be fitted well by a single-temperature model, but two components with temperatures of 0.6 and 1.8 keV are required. The long ionization timescales imply that the gas is close to the ionization equilibrium. The low-temperature component is similar to those seen in other mixed-morphology SNRs. The X-ray luminosity of W28 is ~6 × 1034 ergs s-1, and the estimated X-ray mass is only ~20-25 M☉. A comparison of W28 with other typical mixed-morphology SNRs reveals significant differences in its X-ray properties; W28 has a higher temperature and noticeable spectral variations. W28 belongs to a class of SNRs considered by Chevalier, with a radiative shell interacting with clumpy molecular clouds. X-ray emission at its center is a "fossil" radiation from gas that was shocked early in the evolution of the remnant, and its centrally peaked morphology could have been caused by processes such as evaporation, electron thermal conduction, and mixing induced by various hydrodynamical instabilities, but W28 poses a challenge for existing models of X-ray emission because the evaporation model of White & Long is in conflict with observations, while the presence of temperature variations seems inconsistent with SNR models with efficient thermal conduction.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Rho, J and Borkowski, KJ}, year={2002}, month={Aug}, pages={201–216} }
 @article{li_harrington_borkowski_2002, title={The angular expansion and distance of the planetary nebula BD+30 degrees 3639}, volume={123}, ISSN={["1538-3881"]}, DOI={10.1086/340078}, abstractNote={The WFPC2 camera aboard the Hubble Space Telescope was used to obtain images of the planetary nebula BD +30°3639 at two epochs separated by 5.663 yr. The expansion of the nebula in the Hα and [N II] bands has been measured using several methods. Detailed expansion maps for both emission lines were constructed from nearly 200 almost independent features. There is good agreement between the (independent) Hα and [N II] proper motions. There are clear deviations from uniform radial expansion, with higher expansion rates in regions where the shell is faintest, such as the southwest quadrant. The Space Telescope Imaging Spectrograph was used to obtain echelle spectra in the C II] λ2326 multiplet and the [O II] λ2470 doublet, providing well-resolved expansion velocities at two position angles. From the C II] lines we find that the central velocity split is ±36.3 km s-1 at a position angle (P.A.) of 99° and ±33.5 km s-1 at P.A. 25°. The fainter [O II] doublet does not appear to differ from the C II] multiplet. To determine the distance of BD +30°3639 by comparison of the angular expansion and the spectroscopically determined radial expansion, we must address the problem of the three-dimensional shape of the nebula. We measured the angular expansion along the position of the 99° echelle slit, finding displacements of 4.25 mas yr-1 at the shell edge (2.″47 from the center). If the nebula were spherical, this would imply a distance of 1.80 kpc. But there is evidence that the nebula is elongated along the line of sight, which suggests that the actual distance is less. Radio continuum images from 5 and 15 GHz VLA observations provide information on the extent of the radial elongation. We fit the radio brightness variation and the echelle data by approximating the nebula as an ellipsoid, also making use of the ground-based echelle spectra reported by Bryce & Mellema. Our model has an axial ratio of 1.56, is inclined to the line of sight by 9.°7, and exhibits an expansion in the plane of the sky which is that in the radial direction, leading to a distance of 1.2 kpc. Not all the kinematic data fit this simple model, so the distance must still be regarded as uncertain. Based on the recent model atmosphere of Crowther et al., a distance of 1.2 kpc implies a stellar luminosity of 4250 L⊙. The kinematic age of the nebula, θ/, varies somewhat from region to region. A good average value is 800 yr, while the expansion along the position of the 99° echelle slit gives about 600 yr.}, number={5}, journal={ASTRONOMICAL JOURNAL}, author={Li, JY and Harrington, JP and Borkowski, KJ}, year={2002}, month={May}, pages={2676–2688} }
 @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} }
 @article{fesen_morse_chevalier_borkowski_gerardy_lawrence_van den bergh_2001, title={Hubble Space Telescope WFPC2 imaging of Cassiopeia A}, volume={122}, DOI={10.1086/323539}, abstractNote={The young galactic supernova remnant Cassiopeia A was imaged with Wide Field Planetary Camera 2 (WFPC2) aboard the Hubble Space Telescope through filters selected to capture the complete velocity range of the remnant's main shell in several emission lines. The primary lines detected, along with the specific WFPC2 filters used, were [O III] λλ4959, 5007 (F450W), [N II] λ6583 (F658N), [S II] λλ6716, 6731 + [O II] λλ7319, 7330 + [O I] λλ6300, 6364 (F675W), and [S III] λλ9069, 9532 (F850LP). About three-quarters of the remnant's 4′ diameter main shell was imaged with all four filters in three WFPC2 pointings, with most remaining shell regions imaged in just the F675W filter via three additional pointings. Considerable detail is observed in the reverse-shocked ejecta with typical knot scale lengths of 0.″2–0.″4 (1–2 × 1016 cm). Both bright and faint emission features appear highly clumped with little in the way of a smooth, diffuse emission component detected. Strong differences in [S III] and [O III] line intensities, indicating chemical abundance differences, are also seen, particularly in knots located along the bright northern limb and near the base of the northeastern jet. A line of curved overlapping filaments in the remnant's northwestern rim appears to mark the location of the remnant's reverse shock front in this region. The morphology of some finger-like ejecta structures elsewhere suggest cases in which the reverse shock front is encountering the remnant's clumped ejecta. Large velocity shears (≃1000 km s-1), possibly associated with the formation of these Rayleigh-Taylor–like features, are found in the line profiles of several emission lines (e.g., [S III] λλ9069, 9532 and [Cl II] λ8679) in ground-based, optical spectra of knots near the remnant's center. The [N II] images of the remnant's circumstellar knots (QSFs) reveal them to be 0.″1–0.″6 thick knots and filaments, often with diffuse edges facing away from the center of expansion. Three-color composite images of the whole remnant and certain sections, along with individual filter enlargements of selected regions of the bright optical shell, are presented and discussed.}, number={5}, journal={Astronomical Journal}, author={Fesen, R. A. and Morse, J. A. and Chevalier, R. A. and Borkowski, K. J. and Gerardy, C. L. and Lawrence, S. S. and Van Den Bergh, S.}, year={2001}, pages={2644–2661} }
 @article{borkowski_harrington_2001, title={Kinematics of 1200 kilometer per second jets in He 3-1475}, volume={550}, ISSN={["0004-637X"]}, DOI={10.1086/319812}, abstractNote={Spectroscopic observations of a proto-planetary nebula He 3-1475 with the Space Telescope Imaging Spectrograph reveal the kinematics of its high-velocity (1200 km s-1) jets. The jets are formed at a large (0.15 pc) distance from its central star by collimation of an asymmetric stellar wind in a pair of conical shocks seen in Wide Field Planetary Camera 2 (WFPC2) images. The jets consist of several pairs of knots symmetrically distributed with respect to the central star, with most knots exhibiting a head-tail morphology. Large (up to 650 km s-1) radial velocity gradients are seen within the knots on subarcsecond scales, with velocities decreasing from the knot heads toward their trailing tails. These large velocity gradients are a sign of efficient deceleration of jets by a much slower bipolar outflow. The inclination angle of the bipolar outflow is equal to 40°, based on Doppler shifts of the scattered stellar Hα line. Its velocity is equal to 140 km s-1 at a distance of 0.23 pc from the star and increases monotonically with the radial distance from the star. A comparison of new WFPC2 [N II] λ6584 images with older WFPC2 images reveals expansion of the jets. The measured jet proper motions in combination with their radial velocities imply that He 3-1475 is a Galactic bulge star at a distance of 8 kpc, located 800 pc above the Galactic plane. Its very high luminosity (25,000 L☉) implies that He 3-1475 must be significantly more massive than a typical asymptotic giant branch star within the Galactic bulge, perhaps because of a past mass transfer and/or a merger event in an interacting binary system.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Borkowski, KJ and Harrington, JP}, year={2001}, month={Apr}, pages={778–784} }
 @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} }
 @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{michael_mccray_pun_garnavich_challis_kirshner_raymond_borkowski_chevalier_filippenko_et al._2000, title={Hubble Space Telescope spectroscopy of spot 1 on the circumstellar ring of SN 1987A}, volume={542}, ISSN={["0004-637X"]}, DOI={10.1086/312924}, abstractNote={We present ultraviolet and optical spectra of the first bright spot (P.A. = 29°) on SN 1987A's equatorial circumstellar ring taken with the Space Telescope Imaging Spectrograph. We interpret this spot as the emission produced by radiative shocks that occur where the supernova blast wave strikes an inward protrusion of the ring. The observed line widths and intensity ratios indicate the presence of radiative shocks with velocities ranging from 100 to 250 km s-1 entering dense (≳104 cm-3) gas. These observations, and future observations of the development of the spectra and line profiles, provide a unique opportunity to study the hydrodynamics of radiative shocks.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Michael, E and McCray, R and Pun, CSJ and Garnavich, P and Challis, P and Kirshner, RP and Raymond, J and Borkowski, K and Chevalier, R and Filippenko, AV and et al.}, year={2000}, month={Oct}, pages={L53–L56} }
 @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} }
 @misc{jones_rudnick_jun_borkowski_dubner_frail_kang_kassim_mccray_1998, title={10(51) Ergs: The Evolution of Shell Supernova Remnants}, volume={110}, ISSN={["1538-3873"]}, DOI={10.1086/316122}, abstractNote={This paper reports on the workshop “1051 Ergs: The Evolution of Shell Supernova Remnants,” hosted by the University of Minnesota, 1997 March 23–26. The workshop was designed to address fundamental dynamical issues associated with the evolution of shell supernova remnants and to understand better the relationships between supernova remnants and their environments. Although the title points only to classical, shell SNR structures, the workshop also considered dynamical issues involving X‐ray–filled composite remnants and pulsar‐driven shells, such as that in the Crab Nebula. Approximately 75 observers, theorists, and numerical simulators with wide‐ranging interests attended the workshop. An even larger community helped through extensive on‐line debates prior to the meeting to focus issues and galvanize discussion. In order to deflect thinking away from traditional patterns, the workshop was organized around chronological sessions for “very young,” “young,” “mature,” and “old” remnants, with the implicit recognition that these labels are often difficult to apply. Special sessions were devoted to related issues in plerions and “thermal X‐ray composites.” Controversy and debate were encouraged. Each session also addressed some underlying, general physical themes: How are supernova remnant (SNR) dynamics and structures modified by the character of the circumstellar medium (CSM) and the interstellar medium (ISM), and vice versa? How are magnetic fields generated in SNRs and how do magnetic fields influence SNRs? Where and how are cosmic rays (electrons and ions) produced in SNRs, and how does their presence influence or reveal SNR dynamics? How does SNR blast energy partition into various components over time, and what controls conversion between components? In lieu of a proceedings volume, we present here a synopsis of the workshop in the form of brief summaries of the workshop sessions. The sharpest impressions from the workshop were the crucial and underappreciated roles that environments have on SNR appearance and dynamics and the critical need for broad‐based studies to understand these beautiful but enigmatic objects.}, number={744}, journal={PUBLICATIONS OF THE ASTRONOMICAL SOCIETY OF THE PACIFIC}, author={Jones, TW and Rudnick, L and Jun, BI and Borkowski, KJ and Dubner, G and Frail, DA and Kang, H and Kassim, NE and McCray, R}, year={1998}, month={Feb}, pages={125–151} }
 @article{harrington_lame_borkowski_bregman_tsvetanov_1998, title={Discovery of a 6.4 micron dust feature in hydrogen-poor planetary nebulae}, volume={501}, ISSN={["0004-637X"]}, DOI={10.1086/311439}, abstractNote={Hydrogen-poor planetary nebulae (PNe) Abell 58, Abell 78, and IRAS 15154-5258 were observed with the ISOPHOT low-resolution spectrometer on board the Infrared Space Observatory. Polycyclic aromatic hydrocarbon (PAH) features, commonly seen in H II regions, PNe, and in the diffuse interstellar medium, are absent from their infrared (IR) spectra. A 6.4 μm dust feature is present instead in the IR spectra of Abell 78 and IRAS 15154-5258. We identify this feature as an aromatic C—C stretch feature produced in carbonaceous grains with little or no hydrogen. The 6.4 μm feature is absent in the IR spectrum of the hydrogen-poor material in PN A58, which was ejected nearly 90 yr ago. Our observations of all three PNe are consistent with the hypothesis that the 6.4 μm C—C stretch feature is produced by small, H-free carbonaceous grains, transiently heated to high temperatures by energetic stellar photons. The 6.4 μm feature is not detected in A58 because of the presence of strong IR continuum produced by large, hot grains located close to its central star.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Harrington, JP and Lame, NJ and Borkowski, KJ and Bregman, JD and Tsvetanov, ZI}, year={1998}, month={Jul}, pages={L123–L126} }
 @article{michael_mccray_borkowski_pun_sonneborn_1998, title={High-velocity Ly alpha emission from SNR 1987A}, volume={492}, ISSN={["0004-637X"]}, DOI={10.1086/311097}, abstractNote={The high-velocity Lyα emission from SN 1987A observed with the Space Telescope Imaging Spectrograph (STIS) evidently comes from a reverse shock formed where the outer envelope of SN 1987A strikes ionized gas inside the inner circumstellar ring. The observations can be explained by a simple kinematic model, in which the Lyα emission comes from hydrogen atoms with radial velocity ~15,000 km s-1 crossing a reverse shock in the shape of a slightly prolate ellipsoid with equatorial radius 4.8×1017 cm or ~80% of the distance to the inner surface of the inner ring. N V λλ1239, 1243 emission, if present, has a net luminosity ≲30% times that of the Lyα emission. Future STIS observations should enable us to predict the time of impact with the inner ring and to determine unambiguously whether or not N V emission is present. These observations will offer a unique opportunity to probe the structure of SN 1987A's circumstellar environment and the hydrodynamics and kinetics of very fast shocks.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Michael, E and McCray, R and Borkowski, KJ and Pun, CSJ and Sonneborn, G}, year={1998}, month={Jan}, pages={L143-+} }
 @article{michael_mccray_pun_borkowski_garnavich_challis_kirshner_chevalier_filippenko_fransson_et al._1998, title={New Hubble Space Telescope observations of high-velocity Ly alpha and H alpha in SNR 1987A}, volume={509}, ISSN={["0004-637X"]}, DOI={10.1086/311780}, abstractNote={We describe and model high-velocity (≈ 15,000 km s-1) Lyα and Hα emission from the supernova remnant SNR 1987A seen in 1997 September and October with the Space Telescope Imaging Spectrograph. Part of this emission comes from a reverse shock located at ≈ 75% of the radius of the inner boundary of the inner circumstellar ring and confined within ±30° of the equatorial plane. Departure from axisymmetry in the Lyα and Hα emission correlates with that seen in nonthermal radio emission and reveals an asymmetry in the circumstellar gas distribution. We also see diffuse high-velocity Lyα emission from supernova debris inside the reverse shock that may be due to excitation by nonthermal particles accelerated by the shock.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Michael, E and McCray, R and Pun, CSJ and Borkowski, K and Garnavich, P and Challis, P and Kirshner, RP and Chevalier, R and Filippenko, AV and Fransson, C and et al.}, year={1998}, month={Dec}, pages={L117–L120} }
 @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{harrington_lame_white_borkowski_1997, title={High-resolution imagery of Bd +30 degrees 3639 and its dusty halo}, volume={113}, number={6}, journal={Nanostructured Materials}, author={Harrington, J. P. and Lame, N. J. and White, S. M. and Borkowski, K. J.}, year={1997}, pages={2147} }
 @inproceedings{harrington_borkowski_tsvetanov_1997, title={Modeling hydrogen-deficient planetary nebulae}, volume={180}, DOI={10.1007/978-94-011-5244-0_91}, booktitle={Planetary nebulae: Proceedings of the 180th Symposium of the International Astronomical Union, held in Groningen, The Netherlands, August 26-30, 1996}, publisher={Boston, Mass.: Kluwer Academic Publishers, 1997}, author={Harrington, J. P. and Borkowski, K. J. and Tsvetanov, Z. I.}, editor={H. J. Habing and Lamers, H. J. G. L. M.Editors}, year={1997}, pages={235} }
 @inproceedings{lame_harrington_borkowski_1997, title={Physical conditions in NGC 6543}, volume={180}, DOI={10.1017/s0074180900130712}, abstractNote={We observed NGC 6543 (the Cat's Eye Nebula) with the HST WFPC2 camera in ten narrow-band filters covering a range of emission lines. These images are used to further look at the physical conditions in NGC 6543. We discuss the [O III] electron temperature, ionization state throughout the nebula, and the results of photoionization models of individual structures.}, booktitle={Proceedings of the 180th Symposium of the International Astronomical Union, held in Groningen, The Netherlands, August 26-30, 1996}, publisher={Boston, Mass.: Kluwer Academic Publishers, 1997}, author={Lame, N. J. and Harrington, J. P. and Borkowski, K.}, editor={H. J. Habing and Lamers, H. J. G. L. M.Editors}, year={1997}, pages={252–253} }