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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 seconds 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 years), 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 SN 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 years, 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 years, our 3D model has comparable shocked masses of C+O, IMEs, and IGEs, with about 0.03 $M_\odot$ each. Evolutionary changes appear to be rapid enough that continued monitoring with the Chandra X-ray Observatory may show significant variations., 30 pages, 23 figures. Accepted for publication in ApJS. Animations will be available in online published version

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.

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 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 and the slowest grouping of shocks averaging just 1670 km/s. We compare the H_alpha images from HST with X-ray images from Chandra and mid-IR images from Spitzer, finding a clear anti-correlation 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., Comment: 8 or so pages, half a dozen figures, accepted for publication in ApJ

HAWC J1826-128 is one of the brightest Galactic TeV gamma-ray sources detected by the High Altitude Water Cherenkov (HAWC) Observatory, with photon energies extending up to nearly $\sim$100 TeV. This HAWC source spatially coincides with the H.E.S.S. TeV source HESS J1826-130 and the "Eel" pulsar wind nebula (PWN), which is associated with the GeV pulsar PSR J1826-1256. In the X-ray band, Chandra and XMM-Newton revealed that the Eel PWN is composed of both a compact nebula ($\sim$15") and diffuse X-ray emission ($\sim$6'$\times$2') extending away from the pulsar. Our NuSTAR observation detected hard X-ray emission from the compact PWN up to $\sim$20 keV and evidence of the synchrotron burn-off effect. In addition to the spatial coincidence between HESS J1826-130 and the diffuse X-ray PWN, our multi-wavelength spectral energy distribution (SED) analysis using X-ray and gamma-ray data establishes a leptonic origin of the TeV emission associated with the Eel PWN. Furthermore, our evolutionary PWN SED model suggests (1) a low PWN B-field of $\sim$1 $\mu$G, (2) a significantly younger pulsar age ($t \sim5.7$ kyr) than the characteristic age ($\tau= 14.4$ kyr) and (3) a maximum electron energy of $E_{max} = 2$ PeV. The low B-field as well as the putative supersonic motion of the pulsar may account for the asymmetric morphology of the diffuse X-ray emission. Our results suggest that the Eel PWN may be a leptonic PeVatron particle accelerator powered by the $\sim$6-kyr-old pulsar PSR J1826-1256 with a spin-down power of $3.6 \times 10^{36}$ erg s$^{-1}$., Comment: 19 pages, 12 figures

We present in this paper the hard X-ray view of the pulsar wind nebula in G11.2−0.3 and its central pulsar powered pulsar J1811−1925 as seen by NuSTAR. We complement the data with Chandra for a more complete picture and confirm the existence of a hard, power-law component in the shell with photon index Γ = 2.1 ± 0.1, which we attribute to synchrotron emission. Our imaging observations of the shell show a slightly smaller radius at higher energies, consistent with Chandra results, and we find shrinkage as a function of increased energy along the jet direction, indicating that the electron outflow in the PWN may be simpler than that seen in other young PWNe. Combining NuSTAR with INTEGRAL, we find that the pulsar spectrum can be fit by a power law with Γ = 1.32 ± 0.07 up to 300 keV without evidence of curvature.

We report Chandra observations of the highly asymmetric core-collapse supernova remnant G350.1-0.3. We document expansion over 9 years away from the roughly stationary central compact object, with sky-plane velocities up to $5000 d_{4.5}$ km s$^{-1}$ ($d_{4.5}$ is the distance in units of 4.5 kpc), redshifts ranging from 900 km s$^{-1}$ to 2600 km s$^{-1}$, and three-dimensional space velocities approaching 6000 km s$^{-1}$. Most of the bright emission comes from heavy-element ejecta particularly strong in iron. Iron-enhanced ejecta are seen at 4000 - 6000 km s$^{-1}$, strongly suggesting that the supernova was not a common Type IIP event. While some fainter regions have roughly solar abundances, we cannot identify clear blast-wave features. Our expansion proper motions indicate that G350.1-0.3 is no more than about 600 years old, independent of distance: the third youngest known core-collapse supernova in the Galaxy, and one of the most asymmetric., Comment: 8 pages, 4 figures, ApJ Letters in press, minor revisions

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 upstream magnetic field considerably beyond simple compression. The complex plasma physics processes involved are often parameterized by assuming that shocks put some fraction $\epsilon_e$ of their energy into fast particles, and another fraction $\epsilon_B$ into magnetic field. Modelers of shocks in supernovae, supernova remnants, 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. Supernova remnants (SNRs) can provide such circumstances. Here we summarize results from global fits to spatially integrated emission in six young SNRs, finding $10^{-4} \le \epsilon_e \le 0.05$ and $0.001 \le \epsilon_B \le 0.1$. 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., Comment: 16 pages, 7 figures; accepted for publication in ApJ

We review observations of several classes of neutron-star-powered outflows: pulsar-wind nebulae (PWNe) inside shell supernova remnants (SNRs), PWNe interacting directly with interstellar medium (ISM), and magnetar-powered outflows. We describe radio, X-ray, and gamma-ray observations of PWNe, focusing first on integrated spectral-energy distributions (SEDs) and global spectral properties. High-resolution X-ray imaging of PWNe shows a bewildering array of morphologies, with jets, trails, and other structures. Several of the 23 so far identified magnetars show evidence for continuous or sporadic emission of material, sometimes associated with giant flares, and a few possible "magnetar-wind nebulae" have been recently identified., Comment: 61 pages, 44 figures (reduced in quality for size reasons). Published in Space Science Reviews, "Jets and Winds in Pulsar Wind Nebulae, Gamma-ray Bursts and Blazars: Physics of Extreme Energy Release"

We present a 142-ks Chandra observation of the enigmatic combination supernova remnant G310.6-1.6 consisting of a bright pulsar-wind nebula driven by an energetic pulsar, surrounded by a highly circular, very faint shell with a featureless, probably synchrotron, spectrum. Comparison with an observation 6 years earlier shows no measurable expansion of the shell, though some features in the pulsar-wind nebula have moved. We find an expansion age of at least 2500 yr, implying a current shock velocity less than about 1000 km/s. We place severe upper limits on thermal emission from the shell; if the shell locates the blast wave, a Sedov interpretation would require the remnant to be very young, about 1000 yr, and to have resulted from a dramatically sub-energetic supernova, ejecting << 0.02 M_sun with energy E < 3 x 10^47 erg. Even a merger-induced collapse of a white dwarf to a neutron star, with a low-energy explosion, is unlikely to produce such an event. Other explanations seem equally unlikely., 11 pages, 10 figures; accepted for publication in the Astrophysical Journal

One of the youngest known remnants of a core-collapse supernova (SN) in our Galaxy is G320.4$-$1.2/MSH 15-52 containing an energetic pulsar with a very short (1700 yr) spindown age and likely produced by a stripped-envelope SN Ibc. Bright X-ray and radio emission north of the pulsar overlaps with an H$\alpha$ nebula RCW 89. The bright X-rays there have a highly unusual and quite puzzling morphology, consisting of both very compact thermally emitting knots and much more diffuse emission of nonthermal origin. We report new X-ray observations of RCW 89 in 2017 and 2018 with Chandra that allowed us to measure the motions of many knots and filaments on decade-long time baselines. We identify a fast blast wave with a velocity of $(4000 \pm 500)d_{5.2}$ km/s ($d_{5.2}$ is the distance in units of 5.2 kpc) with a purely nonthermal spectrum, and without any radio counterpart. Many compact X-ray emission knots are moving vary fast, with velocities as high as 5000 km/s, predominantly radially away from the pulsar. Their spectra show that they are Ne- and Mg-rich heavy-element SN ejecta. They have been significantly decelerated upon their recent impact with the dense ambient medium north of the pulsar. We see fast evolution in brightness and morphology of knots in just a few years. Ejecta knots in RCW 89 resemble those seen in Cas A at optical wavelengths in terms of their initial velocities and densities. They might have the same origin, still not understood but presumably related to stripped-envelope SN explosions themselves., Comment: 7 pages, 4 figures, minor revisions, accepted for publication in ApJ Letters

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 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% to 20%, both brightening and fading, a phenomenon previously observed in only two supernova remnants, indicating strong and/or turbulent magnetic fields., 7 pages, 4 figures, 1 Table, minor text changes to match the published version

We present a second epoch of {\it Chandra} observations of the Type Ia LMC 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 yr, 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 likely 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., Comment: Accepted for publication in ApJ Letters

We present the first three-dimensional 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-year 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 non-equilibrium 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 red or blue shift 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 Type Ia supernova explosion models predict a velocity asymmetry in the ejecta. We find no such velocity asymmetries in Tycho, and 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., Accepted for publication in ApJ. Some figures slightly degraded to reduce file size

The youngest Galactic supernova remnant (SNR) G1.9+0.3, produced by a (probable) SN Ia that exploded ~1900 CE, is strongly asymmetric at radio wavelengths, much brighter in the north, but bilaterally symmetric in X-rays. We present the results of X-ray expansion measurements that illuminate the origin of the radio asymmetry. We confirm the mean expansion rate (2011–2015) of 0.58% yr$^{−1}$, but large spatial variations are present. Using the nonparametric "Demons" method, we measure the velocity field throughout the entire SNR, finding that motions vary by a factor of 5, from $0\buildrel{\prime\prime}\over{.} 09$ to $0\buildrel{\prime\prime}\over{.} 44$ yr−1. The slowest shocks are at the outer boundary of the bright northern radio rim, with velocities v s as low as 3600 km s$^{−1}$ (for an assumed distance of 8.5 kpc), much less than v $_{s}$ = 12,000–13,000 km s$^{−1}$ along the X-ray-bright major axis. Such strong deceleration of the northern blast wave most likely arises from the collision of SN ejecta with a much denser than average ambient medium there. This asymmetric ambient medium naturally explains the radio asymmetry. In several locations, significant morphological changes and strongly nonradial motions are apparent. The spatially integrated X-ray flux continues to increase with time. Based on Chandra observations spanning 8.3 yr, we measure its increase at $1.3 \% \pm 0.8 \% $ yr$^{−1}$. The SN ejecta are likely colliding with the asymmetric circumstellar medium ejected by the SN progenitor prior to its explosion., We acknowledge support by NASA through Chandra General Observer Program grants SAO GO5-16069A—C.

We present analytical and numerical studies of models of supernova-remnant (SNR) blast waves expanding into uniform media and interacting with a denser cavity wall, in one spatial dimension. We predict the nonthermal emission from such blast waves: synchrotron emission at radio and X-ray energies, and bremsstrahlung, inverse-Compton emission (from cosmic-microwave-background seed photons, ICCMB), and emission from the decay of $\pi^0$ mesons produced in inelastic collisions between accelerated ions and thermal gas, at GeV and TeV energies. Accelerated particle spectra are assumed to be power-laws with exponential cutoffs at energies limited by the remnant age or (for electrons, if lower) by radiative losses. We compare the results with those from homogeneous ("one-zone") models. Such models give fair representations of the 1-D results for uniform media, but cavity-wall interactions produce effects for which one-zone models are inadequate. We study the time evolution of SNR morphology and emission with time. Strong morphological differences exist between ICCMB and $\pi^0$-decay emission, at some stages, the TeV emission can be dominated by the former and the GeV by the latter, resulting in strong energy-dependence of morphology. Integrated gamma-ray spectra show apparent power-laws of slopes that vary with time, but do not indicate the energy distribution of a single population of particles. As observational capabilities at GeV and TeV energies improve, spatial inhomogeneity in SNRs will need to be accounted for.

The distribution of elements produced in the inner-most layers of a supernova explosion is a key diagnostic for studying the collapse of massive stars. Here we present the results of a 2.4 Ms \textit{NuSTAR} observing campaign aimed at studying the supernova remnant Cassiopeia A (Cas A). We perform spatially-resolved spectroscopic analyses of the $^{44}$Ti ejecta which we use to determine the Doppler shift and thus the three-dimensional (3D) velocities of the $^{44}$Ti ejecta. We find an initial $^{44}$Ti mass of 1.54 $\pm$ 0.21 $\times 10^{-4}$ M$_{\odot}$ which has a present day average momentum direction of 340$^{\circ}$ $\pm$ 15$^{\circ}$ projected on to the plane of the sky (measured clockwise from Celestial North) and tilted by 58$^{\circ}$ $\pm$ 20$^{\circ}$ into the plane of the sky away from the observer, roughly opposite to the inferred direction of motion of the central compact object. We find some $^{44}$Ti ejecta that are clearly interior to the reverse shock and some that are clearly exterior to the reverse shock. Where we observe $^{44}$Ti ejecta exterior to the reverse shock we also see shock-heated iron; however, there are regions where we see iron but do not observe $^{44}$Ti. This suggests that the local conditions of the supernova shock during explosive nucleosynthesis varied enough to suppress the production of $^{44}$Ti in some regions by at least a factor of two, even in regions that are assumed to be the result of processes like $\alpha$-rich freezeout that should produce both iron and titanium., Comment: 14 pages, 13 figures (6 3D animations in the online journal). Accepted for publication in ApJ

Asymmetry is required by most numerical simulations of stellar core-collapse explosions, but the form it takes differs significantly among models. The spatial distribution of radioactive 44Ti, synthesized in an exploding star near the boundary between material falling back onto the collapsing core and that ejected into the surrounding medium, directly probes the explosion asymmetries. CassiopeiaA is a young, nearby, core-collapse remnant from which 44Ti emission has previously been detected but not imaged. Asymmetries in the explosion have been indirectly inferred from a high ratio of observed 44Ti emission to estimated 56Ni emission, from optical light echoes, and from jet-like features seen in the X-ray and optical ejecta. Here we report spatial maps and spectral properties of the 44Ti in Cassiopeia A. This may explain the unexpected lack of correlation between the 44Ti and iron X-ray emission, the latter being visible only in shock-heated material. The observed spatial distribution rules out symmetric explosions even with a high level of convective mixing, as well as highly asymmetric bipolar explosions resulting from a fast-rotating progenitor. Instead, these observations provide strong evidence for the development of low-mode convective instabilities in core-collapse supernovae., Comment: 32 pages, Figures Embedded

Pulsar-wind nebulae emit an extremely broad spectrum of continuum radiation, from low radio frequencies to TeV gamma rays. The part of the spectral energy distribution (SED) from radio through MeV gamma rays is due to synchrotron emission from a distribution of relativistic electrons (or pairs) which can be described by one or more power laws. This spectrum exhibits that particle energy distribution, responsible also for the higher-energy (GeV–TeV) part of the SED, due to inverse-Compton upscattering of one of three photon fields: the synchrotron spectrum, the cosmic microwave background, or ambient optical/infrared photons. However, in a few sources, primary hadrons may produce GeV–TeV gamma rays through the decay of neutral pions produced in inelastic cosmic-ray collisions with thermal gas. The higher-energy end of the particle spectrum, producing synchrotron photons above approximately 10 keV, holds clues to the particle acceleration process. However, its detailed study requires imaging spectroscopy in this energy range, not available until the NuSTAR mission beginning in 2012, which performs true imaging between 3 and 78 keV with ${\sim}1^{\prime }$ angular resolution. I review NuSTAR observations of the first three pulsar-wind nebulae (PWNe) to be examined in this way: the Crab Nebula, G21.5–0.9 and MSH 15–52. All three show spectral structure not previously known: spectral steepening in certain locations and overall source shrinkage with increasing photon energy. The Crab Nebula has different shrinkage rates along the torus and along the northwest counter-jet. The latter rate is similar to that for both the other sources (FWHM $\propto E^{m}$ with $m\sim -0.2$). I discuss implications of these results for models of particle transport in PWNe.

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 which 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 $N_H = 6.7 (4.0, 9.7) \times 10^{21}$ cm$^{-2}$ and photon index $\Gamma = 1.8 (1.4, 2.2)$. For a distance of 4 kpc, the unabsorbed luminosity between 0.5 and 8 keV is $ 1.7^{+0.4}_{-0.3} \times 10^{32}$ erg s$^{-1}$ (90\% confidence intervals). Both $L_X$ and $\Gamma$ are quite typical of pulsars in PWNe. No extended emission is seen; we estimate a conservative $3 \sigma$ upper limit to the surface brightness of any X-ray PWN near the point source to be $3 \times 10^{-17}$ 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 ($\alpha \sim -0.7$), if continued to the X-ray without a break, predicts $L_X\ \rm{(nebula)} \sim 1 \times 10^{33}$ 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., Comment: 5 pages, 3 figures; accepted by ApJ Letters

I outline the dynamical evolution of the shell remnants of supernovae (SNRs), from initial interaction of supernova ejecta with circumstellar material (CSM) through to the final dissolution of the remnant into the interstellar medium (ISM). Supernova ejecta drive a blast wave through any CSM from the progenitor system; as material is swept up, a reverse shock forms in the ejecta, reheating them. This ejecta-driven phase lasts until ten or more times the ejected mass is swept up, and the remnant approaches the Sedov or self-similar evolutionary phase. The evolution up to this time is approximately adiabatic. Eventually, as the blast wave slows, the remnant age approaches the cooling time for immediate post-shock gas, and the shock becomes radiative and highly compressive. Eventually the shock speed drops below the local ISM sound speed and the remnant dissipates. I then review the various processes by which remnants radiate. At early times, during the adiabatic phases, thermal X-rays and nonthermal radio, X-ray, and gamma-ray emission dominate, while optical emission is faint and confined to a few strong lines of hydrogen and perhaps helium. Once the shock is radiative, prominent optical and infrared emission is produced. Young remnants are profoundly affected by interaction with often anisotropic CSM, while even mature remnants can still show evidence of ejecta.

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., Comment: 5 pages, 3 figures, accepted for publication in ApJL

We report new Chandra X-ray observations of the shell supernova remnant (SNR) Kes 75 (G29.7-0.3) containing a pulsar and pulsar-wind nebula (PWN). Expansion of the PWN is apparent across the four epochs, 2000, 2006, 2009, and 2016. We find an expansion rate between 2000 and 2016 of the NW edge of the PWN of 0.249% +/- 0.023% yr^{-1}, for an expansion age R/(dR/dt) of 400 +/- 40 years 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 years. 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 youngest known pulsar. The pulsar PSR J1846-0258 has a spindown-inferred magnetic field of 5 x 10^{13} 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., 15 pages, 9 figures. Accepted by Astrophysical Journal. (Figures 1, 3, 4, and 5 degraded in quality for space reasons.)

We present 24 {mu}m and 70 {mu}m 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 {mu}m to the 24 {mu}m 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, n{sub H}{approx} 2.0 cm{sup -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.

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., Comment: 19 pages, 7 tables, 15 figures

Electromagnetic fields carry energy, momentum, and angular momentum. The momentum density, ϵ0(E×B), accounts (among other things) for the pressure of light. But even static fields can carry momentum, and this would appear to contradict a general theorem that the total momentum of a closed system is zero if its center of energy is at rest. In such cases, there must be some other (nonelectromagnetic) momenta that cancel the field momentum. What is the nature of this “hidden momentum” and what happens to it when the electromagnetic fields are turned off?

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., Comment: Accepted for publication in The Astrophysical Journal Letters

We report on the discovery of extended X-ray emission around the high magnetic field Rotating Radio Transient J1819-1458. Using a 30ks Chandra ACIS-S observation, we found significant evidence for extended X-ray emission with a peculiar shape: a compact region out to 5.5", and more diffuse emission extending out to ~13" from the source. The most plausible interpretation is a nebula somehow powered by the pulsar, although the small number of counts prevents a conclusive answer on the nature of this emission. RRAT J1819-1458's spin-down energy loss rate (Edot~3x10^{32} erg/s) is much lower than that of other pulsars with observed spin-down powered pulsar wind nebulae (PWNe), and implies a rather high X-ray efficiency of eta_{X} = L_(pwn; 0.5-8keV)/Edot~0.2 at converting spin-down power into the PWN X-ray emission. This suggests the need of an additional source of energy rather than the spin-down power alone, such as the high magnetic energy of this source. Furthermore, this Chandra observation allowed us to refine the positional accuracy of RRAT J1819-1458 to a radius of ~0.3", and confirms the presence of X-ray pulsations and the ~1keV absorption line, previously observed in the X-ray emission of this source., Comment: 5 pages, 3 figures; ApJ Letter accepted

Many shell supernova remnants are now known to radiate synchrotron X-rays. Several objects have also been detected in TeV gamma rays. Nonthermal X-rays and gamma rays can be produced in shell remnants by extremely energetic ions and electrons due to decay of π0 mesons produced in inelastic collisions between ions and thermal gas, or by electron synchrotron, bremsstrahlung, or inverse-Compton radiation. Thus observations at X-ray and gamma-ray wavelengths constrain the process of particle acceleration to high energies in the shock waves of supernova remnants. This review examines the relevant characteristics of Type Ia and core-collapse supernovae, the dynamics of their evolution through the Sedov blast-wave phase, the basic physics of diffusive shock acceleration, and the physics of the relevant radiative processes. It also reviews the current status of observations of shell remnants from X-rays to TeV gamma rays, and summarizes what we can learn about particle acceleration.

We present infrared observations of 3C 58 with the Spitzer Space Telescope and the Canada-France-Hawaii Telescope. Using the IRAC camera, we have imaged the entire source resulting in clear detections of the nebula at 3.6 and 4.5 microns. The derived flux values are consistent with extrapolation of the X-ray spectrum to the infrared band, demonstrating that any cooling break in the synchrotron spectrum must occur near the soft X-ray band. We also detect the torus surrounding PSR J0205+6449, the 65 ms pulsar that powers 3C 58. The torus spectrum requires a break between the infrared and X-ray bands, and perhaps multiple breaks. This complex spectrum, which is an imprint of the particles injected into the nebula, has considerable consequences for the evolution of the broadband spectrum of 3C 58. We illustrate these effects and discuss the impact of these observations on the modeling of broadband spectra of pulsar wind nebulae., Comment: 4 pages, 4 figures, accepted for publication in ApJ Letters

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., Accepted by ApJ Letters

We report results from deep observations (~750 ks) of Tycho's supernova remnant (SNR) with NuSTAR. Using these data, we produce narrow-band images over several energy bands to identify the regions producing the hardest X-rays and to search for radioactive decay line emission from 44Ti. We find that the hardest (>10 keV) X-rays are concentrated in the southwest of Tycho, where recent Chandra observations have revealed high emissivity "stripes" associated with particles accelerated to the knee of the cosmic-ray spectrum. We do not find evidence of 44Ti, and we set limits on its presence and distribution within the SNR. These limits correspond to a upper-limit 44Ti mass of M44 < 2.4x10^-4 M_sun for a distance of 2.3 kpc. We perform spatially resolved spectroscopic analysis of sixty-six regions across Tycho. We map the best-fit rolloff frequency of the hard X-ray spectra, and we compare these results to measurements of the shock expansion and ambient density. We find that the highest energy electrons are accelerated at the lowest densities and in the fastest shocks, with a steep dependence of the roll-off frequency with shock velocity. Such a dependence is predicted by models where the maximum energy of accelerated electrons is limited by the age of the SNR rather than by synchrotron losses, but this scenario requires far lower magnetic field strengths than those derived from observations in Tycho. One way to reconcile these discrepant findings is through shock obliquity effects, and future observational work is necessary to explore the role of obliquity in the particle acceleration process., 12 pages, 12 figures, ApJ in press

We present deep ($>$2.4 Ms) observations of the Cassiopeia A supernova remnant with {\it NuSTAR}, which operates in the 3--79 keV bandpass and is the first instrument capable of spatially resolving the remnant above 15 keV. We find that the emission is not entirely dominated by the forward shock nor by a smooth "bright ring" at the reverse shock. Instead we find that the $>$15 keV emission is dominated by knots near the center of the remnant and dimmer filaments near the remnant's outer rim. These regions are fit with unbroken power-laws in the 15--50 keV bandpass, though the central knots have a steeper ($\Gamma \sim -3.35$) spectrum than the outer filaments ($\Gamma \sim -3.06$). We argue this difference implies that the central knots are located in the 3-D interior of the remnant rather than at the outer rim of the remnant and seen in the center due to projection effects. The morphology of $>$15 keV emission does not follow that of the radio emission nor that of the low energy ($$15 keV emission as an open mystery. Even at the forward shock front we find less steepening of the spectrum than expected from an exponentially cut off electron distribution with a single cutoff energy. Finally, we find that the GeV emission is not associated with the bright features in the {\it NuSTAR} band while the TeV emission may be, suggesting that both hadronic and leptonic emission mechanisms may be at work., Comment: 12 pages, 11 figures, accepted for publication in ApJ

Several young supernova remnants exhibit thin X-ray bright rims of synchrotron radiation at their forward shocks. Thin rims require strong magnetic field amplification beyond simple shock compression if rim widths are only limited by electron energy losses. But, magnetic field damping behind the shock could produce similarly thin rims with less extreme field amplification. Variation of rim width with energy may thus discriminate between competing influences on rim widths. We measured rim widths around Tycho's supernova remnant in 5 energy bands using an archival 750 ks Chandra observation. Rims narrow with increasing energy and are well described by either loss-limited or damped scenarios, so X-ray rim width-energy dependence does not uniquely specify a model. But, radio counterparts to thin rims are not loss-limited and better reflect magnetic field structure. Joint radio and X-ray modeling favors magnetic damping in Tycho's SNR with damping lengths ~1--5% of remnant radius and magnetic field strengths ~50--400 $\mu$G assuming Bohm diffusion. X-ray rim widths are ~1% of remnant radius, somewhat smaller than inferred damping lengths. Electron energy losses are important in all models of X-ray rims, suggesting that the distinction between loss-limited and damped models is blurred in soft X-rays. All loss-limited and damping models require magnetic fields $\gtrsim$ 20 $\mu$G, affirming the necessity of magnetic field amplification beyond simple compression., Comment: 17 pages, 9 figures; accepted by ApJ

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 [+0.8, -0.7] kpc. The main shock around the northern rim of the remnant has a typical speed of 1690 km/s 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., Comment: Accepted for publication in the Astrophyscial Journal

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., Comment: 4 pages, 2 figures, submitted to ApJL

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.

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. Subject headings: acceleration of particles — cosmic rays — radiation mechanisms: nonthermal — supernova remnants: individual: G347.3-0.5 — X-rays: general