arXiv subjects

Stephen P. Reynolds

Publications and source records attributed to Stephen P. Reynolds.

At least 55 records · Page 3Linked to original sources

arXiv2014

X-ray Spectral Components in the Afterglow of GRB 130925A

We have identified spectral features in the late-time X-ray afterglow of the unusually long, slow-decaying GRB 130925A using NuSTAR, Swift-XRT, and Chandra. A spectral component in addition to an absorbed power-law is required at $>4σ$ significance, and its spectral shape varies between two observation epochs at $2\times10^5$ and $10^6$ seconds after the burst. Several models can fit this additional component, each with very different physical implications. A broad, resolved Gaussian absorption feature of several keV width improves the fit, but it is poorly constrained in the second epoch. An additive black body or second power-law component provide better fits. Both are challenging to interpret: the blackbody radius is near the scale of a compact remnant ($10^8$ cm), while the second powerlaw component requires an unobserved high-energy cutoff in order to be consistent with the non-detection by Fermi-LAT.

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arXiv2013

Azimuthal Density Variations Around the Rim of Tycho's Supernova Remnant

{\it 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 $\sim 100$ K in the post-shock environment by collisions with energetic electrons and ions. The ratio of the 70 to 24 $μ$m 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, we find an overall gradient in the ambient density surrounding Tycho, with densities 3-10 times higher in the NE than in the SW. 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 ($\sim 0.1-0.2$ 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 SN expanding into a density gradient in the ISM, and find that the overall round shape of the remnant is still easily acheivable, 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 $\sim 0.2$ cm$^{-3}$. Such preshock densities are obtained for highly ($\ga 50$%) porous ISM grains.

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arXiv2013

The Nuclear Spectroscopic Telescope Array (NuSTAR) Mission

The Nuclear Spectroscopic Telescope Array (NuSTAR) mission, launched on 13 June 2012, is the first focusing high-energy X-ray telescope in orbit. NuSTAR operates in the band from 3 -- 79 keV, extending the sensitivity of focusing far beyond the ~10 keV high-energy cutoff achieved by all previous X-ray satellites. The inherently low-background associated with concentrating the X-ray light enables NuSTAR to probe the hard X-ray sky with a more than one-hundred-fold improvement in sensitivity over the collimated or coded-mask instruments that have operated in this bandpass. Using its unprecedented combination of sensitivity, spatial and spectral resolution, NuSTAR will pursue five primary scientific objectives, and will also undertake a broad program of targeted observations. The observatory consists of two co-aligned grazing-incidence X-ray telescopes pointed at celestial targets by a three-axis stabilized spacecraft. Deployed into a 600 km, near-circular, 6degree inclination orbit, the Observatory has now completed commissioning, and is performing consistent with pre-launch expectations. NuSTAR is now executing its primary science mission, and with an expected orbit lifetime of ten years, we anticipate proposing a guest investigator program, to begin in Fall 2014.

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arXiv2012

The First Reported Infrared Emission from the SN 1006 Remnant

We report results of infrared imaging and spectroscopic observations of the SN 1006 remnant, carried out with the Spitzer Space Telescope. The 24 micron image from MIPS 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 micron 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 IRS 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 is lower than that expected for the Galactic ISM -- as has also been observed in the analysis of IR emission from other SNRs but whose cause remains unclear. As with other SN Ia remnants, SN 1006 shows no evidence for dust grain formation in the supernova ejecta.

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arXiv2012

X-ray Emission from Strongly Asymmetric Circumstellar Material in the Remnant of Kepler's Supernova

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 $μ$m emission seen by {\sl 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 $\sim 100^\circ$. We interpret this as evidence for a disk distribution of CSM before the SN, with the line of sight to the observer roughly in the disk plane. We present 2-D 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 AGB star companion.

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arXiv2012

X-ray Proper Motions and Shock Speeds along the Northwest Rim of SN 1006

We report the results of an X-ray proper motion measurement for the NW rim of SN1006, carried out by comparing Chandra observations from 2001 and 2012. The NW limb has predominantly thermal X-ray emission, and it is the only location in SN1006 with significant optical emission: a thin, Balmer-dominated filament. For most of the NW rim, the proper motion is about 0.30 arcsec/yr, essentially the same as has been measured from the H-alpha filament. Isolated regions of the NW limb are dominated by nonthermal emission, and here the proper motion is much higher, 0.49 arcsec/yr, close to the value measured in X-rays along the much brighter NE limb, where the X-rays are overwhelmingly nonthermal. At the 2.2 kpc distance to SN1006, the proper motions imply shock velocities of about 3000 km/s and 5000 km/s in the thermal and nonthermal regions, respectively. A lower velocity behind the H-alpha filament is consistent with the picture that SN1006 is encountering denser gas in the NW, as is also suggested by its overall morphology. In the thermally-dominated portion of the X-ray shell, we also see an offset in the radial profiles at different energies; the 0.5-0.6 keV peak dominated by O VII is closer to the shock front than that of the 0.8-3 keV emission--due to the longer times for heavier elements to reach ionization states where they produce strong X-ray emission.

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arXiv2012

Dust in a Type Ia Supernova Progenitor: Spitzer Spectroscopy of Kepler's Supernova Remnant

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 $μ$m infrared (IR) spectra of the remnant, obtained with the {\it Spitzer Space Telescope}, dominated by emission from warm dust. Broad spectral features at 10 and 18 $μ$m, consistent with various silicate particles, are seen throughout. These silicates were likely formed in the stellar outflow from the progenitor system during the AGB 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 IRS and IRAC 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 $\sim 80-100$ 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 ($n_{0} \sim 50-250$ 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.

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arXiv2011

Pulsar Wind Nebulae with Thick Toroidal Structure

We investigate a class of pulsar wind nebulae that show synchrotron emission from a thick toroidal structure. The best studied such object is the small radio and X-ray nebula around the Vela pulsar, which can be interpreted as the result of interaction of a mildly supersonic inward flow with the recent pulsar wind. Such a flow near the center of a supernova remnant can be produced in a transient phase when the reverse shock reaches the center of the remnant. Other nebulae with a thick toroidal structure are G106.6+2.9 and G76.9+1.0. Their structure contrasts with young pulsar nebulae like the Crab Nebula and 3C 38, which show a more chaotic, filamentary structure in the synchrotron emission. In both situations, a torus-jet structure is present where the pulsar wind passes through a termination shock, indicating the flow is initially toroidal. We suggest that the difference is due to the Rayleigh-Taylor instability that operates when the outer boundary of the nebula is accelerating into freely expanding supernova ejecta. The instability gives rise to mixing in the Crab and related objects, but is not present in the nebulae with thick toroidal regions.

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arXiv2011

RCW 86: A Type Ia Supernova in a Wind-Blown Bubble

We report results from a multi-wavelength analysis of the Galactic SNR RCW 86, the proposed remnant of the supernova of 185 A.D. We report new infrared observations from {\it Spitzer} and {\it WISE}, where the entire shell is detected at 24 and 22 $μ$m. 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 SW and 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 NE 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.

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arXiv2011

Dusty Blastwaves of Two Young LMC Supernova Remnants: Constraints on Postshock Compression

We present results from mid-IR spectroscopic observations of two young supernova remnants (SNRs) in the Large Magellanic Cloud (LMC) done with the {\it Spitzer Space Telescope}. We imaged SNRs B0509-67.5 and B0519-69.0 with {\it 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 {\it Spitzer} IRS data to estimate the density of the postshock 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 omposition assumed, and we explore the effects of differing grain porosity on the model fits. We also analyze archival {\it 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 ISM.

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arXiv2010

Radioactive Scandium in the Youngest Galactic Supernova Remnant G1.9+0.3

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 K-shell 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 $(1 - 7) \times 10^{-5}$ solar masses of 44Ti, in the range predicted for both Type Ia and core-collapse supernovae, but somewhat smaller than the $2 \times 10^{-4}$ solar masses reported for Cas A. The line spectrum indicates supersolar abundances. The Fe emission has a width of about 28,000 km/s, consistent with an age of about 100 yr and with the inferred mean shock velocity of 14,000 km/s 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 scandium, with significant implications for models of nucleosynthesis in Type Ia supernovae.

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arXiv2010

Steady X-Ray Synchrotron Emission in the Northeastern Limb of SN 1006

We investigate time variations and detailed spatial structures of X-ray synchrotron emission in the northeastern limb of SN 1006, using two Chandra observations taken in 2000 and 2008. We extract spectra from a number of small (about 10") regions. After taking account of proper motion and isolating the synchrotron from the thermal emission, we study time variations in the synchrotron emission in the small regions. We find that there are no regions showing strong flux variations. Our analysis shows an apparent flux decline in the overall synchrotron flux of about 4% at high energies, but we suspect that this is mostly a calibration effect, and that flux is actually constant to about 1%. This is much less than the variation found in other remnants where it was used to infer magnetic-field strengths up to 1 mG. We attribute the lack of variability to the smoothness of the synchrotron morphology, in contrast to the small-scale knots found to be variable in other remnants. The smoothness is to be expected for a Type Ia remnant encountering uniform material. Finally we find a spatial correlation between the flux and the cut-off frequency in synchrotron emission. The simplest interpretation is that the cut-off frequency depends on the magnetic-field strength. This would require that the maximum energy of accelerated electrons is not limited by synchrotron losses, but by some other effect. Alternatively, the rate of particle injection and acceleration may vary due to some effect not yet accounted for, such as a dependence on shock obliquity.

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arXiv2009

Deep Chandra Observations of the Crab-like Pulsar Wind Nebula G54.1+0.3 and Spitzer Spectroscopy of the Associated Infrared Shell

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 IRS 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 microns. 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.

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arXiv2008

The First X-Ray Proper-Motion Measurements of the Forward Shock in the Northeastern Limb of SN 1006

We report on the first X-ray proper-motion measurements of the nonthermally-dominated forward shock in the northeastern limb of SN 1006, based on two Chandra observations taken in 2000 and 2008. We find that the proper motion of the forward shock is about 0.48 arcsec/yr and does not vary around the rim within the ~10% measurement uncertainties. The proper motion measured is consistent with that determined by the previous radio observations. The mean expansion index of the forward shock is calculated to be ~0.54 which matches the value expected based on an evolutionary model of a Type Ia supernova with either a power-law or an exponential ejecta density profile. Assuming pressure equilibrium around the periphery from the thermally-dominated northwestern rim to the nonthermally-dominated northeastern rim, we estimate the ambient density to the northeast of SN 1006 to be about 0.085/cm^3.

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arXiv2008

Ejecta, Dust, and Synchrotron Radiation in B0540-69.3: A More Crab-Like Remnant than the Crab

We present near and mid-infrared observations of the pulsar-wind nebula (PWN) B0540-69.3 and its associated supernova remnant made with the {\it Spitzer Space Telescope}. We report detections of the PWN with all four IRAC bands, the 24 $μ$m 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 $μ$m. We show that this dust continuum can be explained by a small amount ($\sim 1-3 \times 10^{-3} \msun$) of dust at a temperature of $\sim 50-65$ 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 ($\sim 20$ 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 $\sim 100-200$ relative to the bulk of the ejecta, and that faster shocks ($\sim 250$ 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 $M_\odot$.

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arXiv2007

X-Ray Emitting Ejecta of Supernova Remnant N132D

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.

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arXiv2007

Spitzer Space Telescope Observations of Kepler's Supernova Remnant: A Detailed Look at the Circumstellar Dust Component

We present 3.6 - 160 micron infrared images of Kepler's supernova remnant (SN1604) obtained with the IRAC and MIPS instruments on the Spitzer Space Telescope. We also present MIPS SED low resolution spectra in the 55 - 95 micron region. The observed emission in the MIPS 24 micron band shows the entire shell. Emission in the MIPS 70 micron and IRAC 8 micron bands is seen only from the brightest regions of 24 micron emission, which also correspond to the regions seen in optical Halpha 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 micron 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 x 10^{-4} solar masses, and infer an original mass of about 3 x 10^{-3} solar masses before grain sputtering. The MIPS 160 micron 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.

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arXiv2006

Dust Destruction in Type Ia Supernova Remnants in the Large Magellanic Cloud

We present first results from an extensive survey of Magellanic Clouds 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 microns 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 microns, and two at 70 microns. A comparison of these images to 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 1-D 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 micron flux ratios only by including sputtering, destroying most grains smaller than 0.03-0.04 microns in radius. We infer total dust masses swept up by the SNR blast waves, before sputtering, of order 0.01 solar masses, several times less than those implied by a dust/gas mass ratio of 0.3 percent as often assumed for the LMC. Substantial dust destruction has implications for gas-phase abundances.

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