@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{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{dyer_goss_kemball_2001, title={Australia Telescope Compact Array observations of the OH star Roberts 22: Resolved images of hydroxyl emission}, volume={121}, ISSN={["0004-6256"]}, DOI={10.1086/320372}, abstractNote={We have imaged the unusual OH/IR object G284-0.8 (Roberts 22) in the OH transitions at 1612, 1665, and 1667 MHz using the Australia Telescope Compact Array in Narrabri, NSW, Australia. The angular resolution of the OH images we present here is 6″ (12,000 AU at a distance of 2 kpc). We find OH emission of extent 1.″5 (2800 AU) located inside the optical bipolar flow region (size 8″) discovered in the early eighties by Allen et al. and imaged recently by Sahai et al. with the Hubble Space Telescope. We present astrometry of Roberts 22, providing registration of the OH and Hubble Space Telescope images to within ∼0.″35. We describe the OH velocity field in Roberts 22 and discuss the kinematic properties of the source and its evolutionary status.}, number={5}, journal={ASTRONOMICAL JOURNAL}, author={Dyer, KK and Goss, WM and Kemball, AJ}, year={2001}, month={May}, pages={2743–2751} }
 @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{lacey_lazio_kassim_duric_briggs_dyer_2001, title={Spatially resolved thermal continuum absorption against supernova remnant W49B}, volume={559}, ISSN={["1538-4357"]}, DOI={10.1086/322372}, abstractNote={We present subarcminute resolution imaging of the Galactic supernova remnant W49B at 74 MHz (25'') and 327 MHz (6''), the former being the lowest frequency at which the source has been resolved. While the 327 MHz image shows a shell-like morphology similar to that seen at higher frequencies, the 74 MHz image is considerably different, with the southwest region of the remnant almost completely attenuated. The implied 74 MHz optical depth (≈1.6) is much higher than the intrinsic absorption levels seen inside two other relatively young remnants, Cas A and the Crab Nebula, nor are natural variations in the relativistic electron energy spectra expected at such levels. The geometry of the absorption is also inconsistent with intrinsic absorption. We attribute the absorption to extrinsic free-free absorption by an intervening cloud of thermal electrons. Its presence has already been inferred from the low-frequency turnover in the integrated continuum spectrum and from the detection of radio recombination lines toward the remnant. Our observations confirm the basic conclusions of those measurements, and our observations have resolved the absorber into a complex of classical H II regions surrounded either partially or fully by low-density H II gas. We identify this low-density gas as an extended H II region envelope (EHE), whose statistical properties were inferred from low-resolution meter- and centimeter-wavelength recombination line observations. Comparison of our radio images with H I and H2CO observations shows that the intervening thermal gas is likely associated with neutral and molecular material as well. This EHE may be responsible for the enhanced radio-wave scattering seen in the general direction of the W49 complex.}, number={2}, journal={ASTROPHYSICAL JOURNAL}, author={Lacey, CK and Lazio, TJW and Kassim, NE and Duric, N and Briggs, DS and Dyer, KK}, year={2001}, month={Oct}, pages={954–962} }
 @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{dyer_reynolds_1999, title={Multifrequency studies of bright radio supernova remnants. III. X-ray and radio}, volume={526}, ISSN={["0004-637X"]}, DOI={10.1086/307985}, abstractNote={Radio-bright, presumably young supernova remnants offer the opportunity of studying strong-shock physics and the nature of the interaction of ejected material with the surrounding medium. The relation between radio and X-ray morphology varies considerably among supernova remnants, with important implications for the physics of the emission processes at different wavelengths. We use Very Large Array (VLA) and Roentgen Satellite (ROSAT) images of the radio-bright supernova remnant 3C 397 (G41.1-0.3) to examine the shock structure in both thermal X-ray emission and nonthermal radio emission. The unusual rectangular morphology can be seen in VLA maps at 20 and 6 cm wavelength at a resolution of 6'' and in ROSAT HRI images. The X-ray images resemble the radio strongly, except for a small, possibly unresolved X-ray hot spot near the center. There is no variation in the X-ray hardness ratio from ROSAT Position Sensitive Proportional Counter data across the remnant, suggesting that at least between 0.4 and 2 keV the interior emission is not different in character from that in the bright shell regions. The remnant is unpolarized at 20 cm and has a mean fractional polarization of 1.5% ± 0.1% at 6 cm. The polarized flux, and polarized fraction, peak inside the remnant at a location not coincident with either an internal maximum in total-intensity radio emission or with the X-ray hot spot. Spectral index maps between 6 and 20 cm do not show any systematic differences associated with interior emission; there appears to be no "plerionic" or pulsar-driven component in 3C 397, at least as normally characterized by high polarization and a flat radio spectrum. Spectral index values spread about the mean by about Δα ~ 0.2, a result consistent with previous work. The steep total-intensity profile off the southwest edge of 3C 397 allows an inference of the upstream electron diffusion coefficient and implies a mean free path for electron scattering shorter than in the general interstellar medium but longer than that similarly inferred for Tycho and SN 1006. A simple analysis based on the observed X-ray flux gives an estimate of the mean density in 3C 397 of about 4 cm-3, which would also be enough to depolarize the 20 cm emission completely, as observed. The remnant age is then of order 103 yr, and the current shock velocity is about 1600 km s-1. Finally, we speculate on possible mechanisms producing the X-ray hot spot.}, number={1}, journal={ASTROPHYSICAL JOURNAL}, author={Dyer, KK and Reynolds, SP}, year={1999}, month={Nov}, pages={365–384} }