arXiv subjects

S. P. Reynolds

Publications and source records attributed to S. P. Reynolds.

At least 37 records · Page 2Linked to original sources

arXiv2007

A Deep Chandra Observation of Kepler's Supernova Remnant: A Type Ia Event with Circumstellar Interaction

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 K alpha 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 rations; 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 about 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.

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arXiv2007

Observations of X-rays and Thermal Dust Emission from the Supernova Remnant Kes 75

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 microns 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 M_dust/M_gas ~0.001. 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.

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arXiv2006

Dense, Fe-rich Ejecta in Supernova Remnants DEM L238 and DEM L249: A New Class of Type Ia Supernova?

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 solar masses 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 3x10^50 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.

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arXiv2006

A New Young Galactic Supernova Remnant Containing a Compact Object: G15.9+0.2

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 $n_et = 6 (4, 9) \times 10^{10}$ 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 are not well constrained by the data. Various estimates give a remnant age of order $10^3$ 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 $Γ\sim$ 4 power law or a $kT \sim$ 0.4 keV blackbody. The spectrum is absorbed by a H column density $N_H \sim 4 \times 10^{22}$ cm$^{-2}$ similar to that required for the remnant shell. The implied 2--9.5 keV source luminosity is about $10^{33}$ 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 (CCO).

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arXiv2006

Dust Destruction in Fast Shocks of Core-Collapse Supernova Remnants in the Large Magellanic Cloud

We report observations with the MIPS instrument aboard the {\it Spitzer Space Telescope} (SST) of four supernova remnants (SNRs) believed to be the result of core-collapse SNe: 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 $μ$m and in part at 70 $μ$m. Comparisons with {\it 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 70/24 $μ$m 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.

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arXiv2006

X-Ray Observations of SN 1006 with Integral

The remnant of the supernova of 1006 AD, the remnant first showing evidence for the presence of X-ray synchrotron emission from shock-accelerated electrons, was observed for ~1000 ksec with INTEGRAL for the study of electron acceleration to very high energies. The aim of the observation was to characterize the synchrotron emission, and attempt to detect non-thermal bremsstrahlung, using the combination of IBIS and JEM-X spatial and spectral coverage. The source was detected with JEM-X between 2.4 and 8.4 keV bands, and not detected with either ISGRI or SPI above 20 keV. The ISGRI upper limit is about a factor of four above current model predictions, but confirms the presence of steepening in the power-law extrapolated from lower energies (< 4 keV).

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arXiv2006

Searching for annihilation radiation from SN 1006 with SPI on INTEGRAL

Historical Type Ia supernovae are a leading candidate for the source of positrons observed through their diffuse annihilation emission in the Galaxy. However, search for annihilation emission from individual Type Ia supernovae has not been possible before the improved sensitivity of \integral. The total 511 keV annihilation flux from individual SNe Ia, as well as their contribution to the overall diffuse emission, depends critically on the escape fraction of positrons produced in $^{56}$Co decays. Late optical light curves suggest that this fraction may be as high as 5%. We searched for positron annihilation radiation from the historical Type Ia supernova SN 1006 using the SPI instrument on \integral. We did not detect significant 511 keV line emission, with a 3$σ$ flux upper limit of 0.59 x 10$^{-4}$ ergs cm^-2 s^-1 for \wsim 1 Msec exposure time, assuming a FWHM of 2.5 keV. This upper limit corresponds to a 7.5% escape fraction, 50% higher than the expected 5% escape scenario, and rules out the possibility that Type Ia supernovae produce all of the positrons in the Galaxy (~ 12% escape fraction), if the mean positron lifetime is less than 10$^{5}$ years. Future observations with \integral will provide stronger limits on the escape fraction of positrons, the mean positron lifetime, and the contribution of Type Ia supernovae to the overall positron content of the Galaxy.

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arXiv2005

Discovery of the X-ray Counterpart to the Rotating Radio Transient J1819--1458

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 $\sim 5\times10^{13}$ G) member of a new class of unusual pulsar-like objects discovered by their bursting activity at radio wavelengths. The position of RRAT J1819--1458 was serendipitously observed by the {\sl Chandra} ACIS-I camera in 2005 May. At that position we have discovered a pointlike source, CXOU J181934.1--145804, with a soft spectrum well fit by an absorbed blackbody with $N_H = 7^{+7}_{-4} \times 10^{21}$ cm$^{-2}$ and temperature $kT=0.12 \pm 0.04$ keV, having an unabsorbed flux of $\sim2 \times 10^{-12}$ 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 five-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.

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arXiv2003

Separating Thermal and Non-Thermal X-Rays in Supernova Remnants II: Spatially Resolved Fits to SN1006 AD

We present a spatially resolved spectral analysis of full ASCA observations of the remnant of the supernova of 1006 AD. 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 gamma-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.

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arXiv2003

A High-Resolution Study of Nonthermal Radio and X-Ray Emission from SNR G347.3-0.5

G347.3-0.5 is one of three shell-type supernova remnants in the Galaxy whose X-ray spectrum is dominated by nonthermal emission. This puts G347.3-0.5 in the small, but growing class of SNRs for which the X-ray emission reveals directly the presence of extremely energetic electrons accelerated by the SNR shock. We have obtained new high-resolution X-ray and radio data on G347.3-0.5 using the Chandra X-ray Observatory and the Australia Telescope Compact Array (ATCA) respectively. The bright northwestern peak of the SNR seen in ROSAT and ASCA images is resolved with Chandra into bright filaments and fainter diffuse emission. These features show good correspondence with the radio morphological structure, providing strong evidence that the same population of electrons is responsible for the synchrotron emission in both bands in this part of the remnant. Spectral index information from both observations is presented. We found significant difference in photon index value between bright and faint regions of the SNR shell. Spectral properties of these regions support the notion that efficient particle acceleration is occurring in the bright SNR filaments. We report the detection of linear radio polarization towards the SNR, which is most ordered at the northwestern shell where particle acceleration is presumably occurring. Using our new Chandra and ATCA data we model the broad-band emission from G347.3-0.5 with the synchrotron and inverse Compton mechanisms and discuss the conditions under which this is a plausible scenario.

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arXiv2003

Microphysics of Shock Acceleration from Observations of X-Ray Synchrotron Emission from Supernova Remnants

Recent observations of non-thermal X-rays from supernova remnants have been attributed to synchrotron radiation from the loss-steepened tail of a non-thermal distribution of electrons accelerated at the remnant blast wave. In diffusive shock acceleration, the maximum particle energy depends on the energy-dependence of the diffusion coefficient kappa. Most work on shock acceleration has made the plausible assumption that kappa is proportional to the particle gyroradius, implying a wavelength-independent spectrum of MHD turbulence, where Kolmogorov or Kraichnan spectra might be more physically plausible. I derive the wave-spectrum-dependence of the maximum electron energy resulting from limitations due to radiative (synchrotron and inverse-Compton) losses and to finite remnant age (or size). I then exhibit calculations of synchrotron X-ray spectra, and model images, for supernova remnants as a function of spectrum, and compare to earlier results. Spectra can be considerably altered, and images are dramatically different, for Kolmogorov or Kraichnan spectra of turbulence. The predicted images are quite unlike observed remnants, suggesting that the turbulence near SNRs is generated by the high-energy particles themselves.

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arXiv2003

Discovery of a New Pulsar Wind Nebula in the Large Magellanic Cloud

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 non-thermal 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 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.

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arXiv2000

Separating Thermal and Nonthermal X-Rays in Supernova Remnants I: Total Fits to SN 1006 AD

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 3.1E17 Hz, at which the spectrum has dropped about a factor of 6 below a powerlaw 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 post-shock magnetic field strength of about 9 microGauss, implying (for a compression ratio of 4) an upstream magnetic field of 3 microGauss, and fixing the current energy content in relativistic electrons at about 7E48 ergs, resulting in a current electron-acceleration efficiency of about 5%. This total energy is about 100 times the energy in the magnetic field. 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.

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arXiv2000

Dynamics of Fe-Ni Bubbles in Young Supernova Remnants

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 3-dimensional hydrodynamic simulations of a single Fe bubble immersed in an outer ejecta envelope, and compare the results with previous work on shock-cloud interactions. 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.

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arXiv2000

Supernova Remnants in the Sedov Expansion Phase: Thermal X-Ray Emission

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 succesfully 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.

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arXiv2000

A Broadband X-Ray Study of the Supernova Remnant 3C 397

We present an X-ray study of the radio bright supernova remnant (SNR) 3C 397 with ROSAT, ASCA, and RXTE. A central X-ray spot seen with the ROSAT High-Resolution Imager hints at the presence of a pulsar-powered component, and gives this SNR a composite X-ray morphology. Combined ROSAT and ASCA imaging show that the remnant is highly asymmetric, with its hard X-ray emission peaking at the western lobe. The spectrum of 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 spectrum, and at least two components are required. We use a set of non-equilibrium ionization (NEI) models (Borkowski et al. in preparation). The temperatures from the soft and hard components are 0.2 keV and 1.6 keV respectively. The corresponding ionization time-scales $n_0 t$ ($n_0$ being the pre-shock hydrogen density) are 6 $\times 10^{12}$ cm$^{-3}$ s and 6 $\times$ 10$^{10}$ cm$^{-3}$ s, respectively. The spectrum obtained with the Proportional Counter Array (PCA) of RXTE is contaminated by emission from the Galactic ridge, with only $\sim$ 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 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. Spatially resolved spectroscopy (with CHANDRA and XMM) is needed to differentiate between the two scenarios, and address the nature of the mysterious radio-quiet X-ray hot spot.

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arXiv2000

Thermal and Nonthermal X-Ray Emission in SNR RCW 86

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 blast wave velocity of 800 km/s, the shock ionization age of 1-3x10^11 s/cm^3, and the break in nonthermal spectra at 2-4x10^16 Hz. The strength of nonthermal continuum correlates well with the radio brightness in the bright SW section of the remnant. This is convincing evidence for X-ray synchrotron emission in RCW 86.

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arXiv2000

New Models for X-Ray Synchrotron Radiation from the Remnant of Supernova 1006 AD

Galactic cosmic rays up to energies of around 10^15 eV are assumed to originate in supernova remnants (SNRs). The shock wave of a young SNR like SN 1006 AD can accelerate electrons to energies greater than 1 TeV, where they can produce synchrotron radiation in the X-ray band. A new model (SRESC) designed to model synchrotron X-rays from Type Ia supernovae can constrain values for the magnetic-field strength and electron scattering properties, with implications for the acceleration of the unseen ions which dominate the cosmic-ray energetics. New observations by ASCA, ROSAT, and RXTE have provided enormously improved data, which now extend to higher X-ray energies. These data allow much firmer constraints. We will describe model fits to these new data on SN 1006 AD, emphasizing the physical constraints that can be placed on SNRs and on the cosmic-ray acceleration process.

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