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We present the results of ALMA band 3 observations of a nearby type Ic supernova (SN) 2020oi. Under the standard assumptions on the SN-circumstellar medium (CSM) interaction and the synchrotron emission, the data indicate that the CSM structure deviates from a smooth distribution expected from the steady-state mass loss in the very vicinity of the SN (~10^{15} cm), which is then connected to the outer smooth distribution (~10^{16} cm). This structure is further confirmed through the light curve modeling of the whole radio data set as combined with data at lower frequency previously reported. Being an explosion of a bare carbon-oxygen (C+O) star having a fast wind, we can trace the mass-loss history of the progenitor of SN 2020oi in the final year. The inferred non-smooth CSM distribution corresponds to fluctuations on the sub-year time scale in the mass-loss history toward the SN explosion. Our finding suggests that the pre-SN activity is likely driven by the accelerated change in the nuclear burning stage in the last moments just before the massive star's demise. The structure of the CSM derived in this study is beyond the applicability of the other methods at optical wavelengths, highlighting an importance and uniqueness of quick follow-up observations of SNe by ALMA and other radio facilities., Instituto de Astrofísica de La Plata

We present a new model-independent (applicable for a broad range of equations of state) analysis of the neutrino emissivity due to triplet neutron pairing in neutron star cores. We find that the integrated neutrino luminosity of the Cooper Pair Formation (CPF) process can be written as a product of two factors. The first factor depends on the neutron star mass, radius and maximal critical temperature of neutron pairing in the core, $T_{Cn \mathrm{max}}$, but not on the particular superfluidity model; it can be expressed by an analytical formula valid for many nucleon equations of state. The second factor depends on the shape of the critical temperature profile within the star, the ratio of the temperature $T$ to $T_{Cn \mathrm{max}}$, but not on the maximal critical temperature itself. While this second factor depends on the superfluidity model, it obeys several model-independent constraints. This property allows one to analyse the thermal evolution of neutron stars with superfluid cores without relying on a specific model of their interiors. The constructed expressions allow us to perform a self-consistent analysis of spectral data and neutron star cooling theory. We apply these findings to the cooling neutron star in the Cassiopeia A supernova remnant using 14 sets of observations taken over 19 years. We constrain $T_{Cn\mathrm{max}}$ to the range of $ (5-10)\times 10^8$ K. This value depends weakly on the equation of state and superfluidity model, and will not change much if cooling is slower than the current data suggest. We also constrain the overall efficiency of the CPF neutrino luminosity., 18 pages, 14 figures, 6 tables; accepted for publication in MNRAS

Massive stars can shed material via steady, line-driven winds, eruptive outflows, or mass-transfer onto a binary companion. In the case of single stars, the mass is deposited by the stellar wind into the nearby environment. After the massive star explodes, the stellar ejecta interact with this circumstellar material (CSM), often-times resulting in bright X-ray line emission from both the shock-heated CSM and ejecta. The amount of material lost by the progenitor, the mass of ejecta, and its energetics all impact the bulk spectral characteristics of this X-ray emission. Here we present a grid of core-collapse supernova remnant models derived from models for massive stars with zero age main sequence masses of $\sim$ 10 - 30 M$_\odot$ evolved from the pre-main sequence stage with wind-driven mass-loss. Evolution is handled by a multi-stage pipeline of software packages. First, we use mesa (Modules for Experiments in Stellar Astrophysics) to evolve the progenitors from pre-main sequence to iron core collapse. We then use the Supernova Explosion Code (snec) to explode the mesa models, and follow them for the first 100 days following core-collapse. Finally, we couple the snec output, along with the CSM generated from mesa mass-loss rates, into the Cosmic-Ray Hydrodynamics code (ChN) to model the remnant phase to 7000 years post core-collapse. At the end of each stage, we compare our outputs with those found in the literature, and we examine any qualitative and quantitative differences in the bulk properties of the remnants and their spectra based on the initial progenitor mass, as well as mass-loss history., 19 pages, 25 figures, 1 table, submitted to ApJ

We report our measurements of the bulk radial velocity from a sample of small, metal-rich ejecta knots in Kepler's Supernova Remnant (SNR). We measure the Doppler shift of the He-like Si K$\alpha$ line center energy in the spectra of these knots based on our $Chandra$ High-Energy Transmission Grating Spectrometer (HETGS) observation to estimate their radial velocities. We estimate high radial velocities of up to $\sim$ 8,000 km s$^{-1}$ for some of these ejecta knots. We also measure proper motions for our sample based on the archival $Chandra$ Advanced CCD Imaging Spectrometer (ACIS) data taken in 2000, 2006, and 2014. Our measured radial velocities and proper motions indicate that some of these ejecta knots are almost freely-expanding after $\sim$ 400 years since the explosion. The fastest moving knots show proper motions up to $\sim$ 0.2 arcseconds per year. Assuming that these high velocity ejecta knots are traveling ahead of the forward shock of the SNR, we estimate the distance to Kepler's SNR $d$ $\sim$ 4.4 to 7.5 kpc. We find that the ejecta knots in our sample have an average space velocity of $ v_{s} \sim$ 4,600 km s$^{-1}$ (at a distance of 6 kpc). We note that 8 out of the 15 ejecta knots from our sample show a statistically significant (at the 90$\%$ confidence level) redshifted spectrum, compared to only two with a blueshifted spectrum. This may suggest an asymmetry in the ejecta distribution in Kepler's SNR along the line of sight, however a larger sample size is required to confirm this result., Comment: Accepted by ApJ, 17 pages, 7 figures, 3 tables

Diffuse interstellar bands (DIBs) are absorption features commonly observed in optical/near-infrared spectra of stars and thought to be associated with polyatomic molecules that comprise a significant reservoir of organic material in the universe. However, the central wavelengths of almost all DIBs do not correspond with electronic transitions of known atomic or molecular species and the specific physical nature of their carriers remains inconclusive despite decades of observational, theoretical, and experimental research. It is well established that DIB carriers are located in the interstellar medium, but the recent discovery of time-varying DIBs in the spectra of the extragalactic supernova SN2012ap suggests that some may be created in massive star environments. Here we report evidence of short time-scale (10-60 d) changes in DIB absorption line substructure toward 3 of 17 massive stars observed as part of a pathfinder survey of variable DIBs conducted with the 1.5-m Tillinghast telescope and Tillinghast Reflector Echelle Spectrograph (TRES) at Fred L. Whipple Observatory. The detections are made in high-resolution optical spectra (R = 44000) having signal-to-noise ratios of 5-15 around the 5797 and 6614 angstrom features, and are considered significant but requiring further investigation. We find that these changes are potentially consistent with interactions between stellar winds and DIB carriers in close proximity. Our findings motivate a larger survey to further characterize these variations and may establish a powerful new method for probing the poorly understood physical characteristics of DIB carriers., Comment: 11 pages, 7 figures, accepted for publication in MNRAS

The study of how neutron stars cool over time can provide invaluable insights into fundamental physics such as the nuclear equation of state and superconductivity and superfluidity. A critical relation in neutron star cooling is the one between observed surface temperature and interior temperature. This relation is determined by the composition of the neutron star envelope and can be influenced by the process of diffusive nuclear burning (DNB). We calculate models of envelopes that include DNB and find that DNB can lead to a rapidly changing envelope composition which can be relevant for understanding the long-term cooling behavior of neutron stars. We also report on analysis of the latest temperature measurements of the young neutron star in the Cassiopeia A supernova remnant. The 13 Chandra observations over 18 years show that the neutron star's temperature is decreasing at a rate of 2-3 percent per decade, and this rapid cooling can be explained by the presence of a proton superconductor and neutron superfluid in the core of the star., Comment: 7 pages, 7 figures; to appear in the AIP Conference Proceedings of the Xiamen-CUSTIPEN Workshop on the EOS of Dense Neutron-Rich Matter in the Era of Gravitational Wave Astronomy (January 3-7, 2019, Xiamen, China)

We measure the expansion of the forward shock of the Small Magellanic Cloud supernova remnant 1E\,0102.2-7219 in X-rays using Chandra X-Ray Observatory on-axis Advanced CCD Imaging Spectromete(ACIS) observations from 1999-2016. We estimate an expansion rate of 0.025%\pm0.006%yr^{-1} and a blast-wave velocity of 1.61\pm0.37\times10^3 km s^{-1}. Assuming partial electron-ion equilibration via Coulomb collisions and cooling due to adiabatic expansion, this velocity implies a postshock electron temperature of 0.84\pm0.20 keV which is consistent with the estimate of 0.68\pm0.05 keV based on the X-ray spectral analysis. We combine the expansion rate with the blast wave and reverse shock radii to generate a grid of one-dimensional models for a range of ejecta masses (2-6 \msol) to constrain the explosion energy, age, circumstellar density, swept-up mass, and unshocked-ejecta mass. We find acceptable solutions for a constant density ambient medium and for an r^{-2} power-law profile (appropriate for a constant progenitor stellar wind). For the constant density case, we find an age of \sim 1700 yr, explosion energies 0.87-2.61\times10^{51} erg, ambient densities 0.85-2.54 amu cm^{-3}, swept-up masses 22-66 \msol, and unshocked-ejecta masses 0.05-0.16 \msol. For the power-law density profile, we find an age of \sim 2600 yr, explosion energies 0.34-1.02\times10^{51} erg, densities 0.22-0.66 amu cm^{-3} at the blast wave, swept-up masses 17-52 \msol, and unshocked-ejecta masses 0.06-0.18 \msol. Assuming the true explosion energy was 0.5-1.5\times10^{51} erg, ejecta masses 2-3.5 \msol are favored for the constant density case and 3-6 \msol for the power-law case. The unshocked-ejecta mass estimates are comparable to Fe masses expected in core-collapse supernovae with progenitor mass 15.0-40.0 \msol, offering a possible explanation for the lack of Fe emission observed in X-rays.

We present new proper motion measurements of optically emitting oxygen-rich knots of supernova remnant 1E 0102.2-7219 (E0102), which are used to estimate the remnant's center of expansion and age. Four epochs of high resolution Hubble Space Telescope images spanning 19 yr were retrieved and analyzed. We found a robust center of expansion of alpha=1:04:02.48 and delta=-72:01:53.92 (J2000) with 1-sigma uncertainty of 1.77 arcseconds using 45 knots from images obtained with the Advanced Camera for Surveys using the F475W filter in 2003 and 2013 having the highest signal-to-noise ratio. We also estimate an upper limit explosion age of 1738 +/- 175 yr by selecting knots with the highest proper motions, that are assumed to be the least decelerated. We find evidence of an asymmetry in the proper motions of the knots as a function of position angle. We conclude that these asymmetries were most likely caused by interaction between E0102's original supernova blast wave and an inhomogeneous surrounding environment, as opposed to intrinsic explosion asymmetry. The observed non-homologous expansion suggests that the use of a free expansion model inaccurately offsets the center of expansion and leads to an overestimated explosion age. We discuss our findings as they compare to previous age and center of expansion estimates of E0102 and their relevance to a recently identified candidate central compact object., 14 pages, 11 figures, revised according to referee comments and resubmitted to ApJ

We present an improved proper motion measurement of the central compact object RX J0822-4300, located in the supernova remnant Puppis A. By employing a new data set taken in February 2019 by the High Resolution Camera aboard the Chandra X-ray Observatory, we approximately double the available temporal baseline for our analysis to slightly more than 19 years (7000 days). We correct for the astrometric inaccuracy of Chandra using calibrator stars with known optical positions that are detected in all observations. Thereby, we obtain absolute positions of RX J0822-4300 accurate to around $0.1^{\prime\prime}$ and from these a new best estimate for its total proper motion of $\mu_{\rm tot}= (80.4 \pm 7.7)\,\rm{mas\,yr}^{-1}$. For a remnant distance of 2 kpc, this corresponds to a projected kick velocity of $(763 \pm 73)\, \rm{km\,s}^{-1}$ at a position angle of $\phi_0 = (247.8 \pm 4.4)^{\circ}$. The proper motion measurement of RX J0822-4300 is used for discussing the kinematic age of Puppis A., Comment: 15 pages, 7 figures, Accepted for publication in ApJ

We present a three-dimensional kinematic reconstruction of the optically-emitting, oxygen-rich ejecta of supernova remnant N132D in the Large Magellanic Cloud. Data were obtained with the 6.5 m Magellan telescope in combination with the IMACS+GISMO instrument and survey [O III] $\lambda\lambda$4959,5007 line emission in a ${\sim}$3$^{\prime}~\times$ 3$^{\prime}$ region centered on N132D. The spatial and spectral resolution of our data enable detailed examination of the optical ejecta structure. The majority of N132D's optically bright oxygen ejecta are arranged in a torus-like geometry tilted approximately 28$^{\circ}$ with respect to the plane of the sky. The torus has a radius of 4.4 pc ($D_{\rm LMC}$/50 kpc), exhibits a blue-shifted radial velocity asymmetry of $-3000$ to $+2300$ km s$^{-1}$, and has a conspicuous break in its circumference. Assuming homologous expansion from the geometric center of O-rich filaments, the average expansion velocity of 1745 km s$^{-1}$ translates to an age since explosion of 2450 $\pm$ 195 yr. A faint, spatially-separated "runaway knot" (RK) with total space velocity of 3650 km s$^{-1}$ is nearly perpendicular to the torus plane and coincident with X-ray emission that is substantially enhanced in Si relative to the LMC and N132D's bulk ejecta. These kinematic and chemical signatures suggest that the RK may have had its origin deep within the progenitor star. Overall, the main shell morphology and high-velocity, Si-enriched components of N132D have remarkable similarity with that of Cassiopeia A, which was the result of a Type IIb supernova explosion. Our results underscore the need for further observations and simulations that can robustly reconcile whether the observed morphology is dominated by explosion dynamics or shaped by interaction with the environment., Comment: 18 pages, 12 figures, accepted for publication in ApJ; 3D animation of N132D can found at: http://www.physics.purdue.edu/kaboom/n132d-anim.mp4

Cassiopeia A (Cas A) is one of the best studied young Galactic supernova remnants. While providing a rare opportunity to study in detail the remnant of a Type IIb supernova, questions remain regarding the nature of its progenitor, its mass-loss history, and its pre-SN evolution. Here we present an optical investigation of the circumstellar environment around Cas A and find clumpy and filamentary Halpha emission nebulosities concentrated 10-15 pc (10-15 arcminutes) to the north and east. First reported by Minkowski as a faint H II region, these nebulosities exhibit distinct morphological and spectroscopic properties relative to the surrounding diffuse emissions. Compared to neighboring H II regions, these nebulae show stronger [N II] 6548, 6583 A and [S II] 6716, 6731 A emissions relative to Halpha. We show that Cas A's highest-velocity ejecta knots are interacting with some of the closest projected emission nebulae, thus providing strong evidence that these nebulae lie at the same distance as the remnant. We interpret these surrounding nebulosities to be the remains of the progenitor's red supergiant wind which accumulated against the southern edge of a large extended H II region located north of Cas A. Our findings are consistent with the view that Cas A's progenitor underwent considerable mass-loss, first from a fast main-sequence wind, then from a slower, clumpy red supergiant wind, and finally from a brief high-velocity wind, like that from a yellow supergiant., Comment: 19 pages, 9 figures, accepted to ApJ

Type Ia supernovae originate from the explosion of carbon-oxygen white dwarfs in binary systems, but the exact nature of their progenitors remains elusive. The bulk properties of Type Ia supernova remnants, such as the radius and the centroid energy of the Fe K$\alpha$ blend in the X-ray spectrum, are determined by the properties of the supernova ejecta and the ambient medium. We model the interaction between Chandrasekhar and sub-Chandrasekhar models for Type Ia supernova ejecta and a range of uniform ambient medium densities in one dimension up to an age of 5000 years. We generate synthetic X-ray spectra from these supernova remnant models and compare their bulk properties at different expansion ages with X-ray observations from \textit{Chandra} and \textit{Suzaku}. We find that our models can successfully reproduce the bulk properties of most observed remnants, suggesting that Type Ia SN progenitors do not modify their surroundings significantly on scales of a few pc. Ambient medium density and expansion age are the main contributors to the diversity of the bulk properties in our models. Chandrasekhar and sub-Chandrasekhar progenitors make similar predictions for the bulk remnant properties, but detailed fits to X-ray spectra have the power to discriminate explosion energetics and progenitor scenarios., Comment: 22 pages, 14 figures, published in the ApJ

We present the first extensive radio to gamma-ray observations of a fast-rising blue optical transient (FBOT), AT2018cow, over its first ~100 days. AT2018cow rose over a few days to a peak luminosity $L_{pk}\sim4\times 10^{44}$ erg/s exceeding those of superluminous supernovae (SNe), before declining as $\propto t^{-2}$. Initial spectra at $\lesssim 15$ days were mostly featureless and indicated large expansion velocities v~0.1c and temperatures reaching 30000 K. Later spectra revealed a persistent optically-thick photosphere and the emergence of H and He emission features with v~sim 4000 km/s with no evidence for ejecta cooling. Our broad-band monitoring revealed a hard X-ray spectral component at $E\ge 10$ keV, in addition to luminous and highly variable soft X-rays, with properties unprecedented among astronomical transients. An abrupt change in the X-ray decay rate and variability appears to accompany the change in optical spectral properties. AT2018cow showed bright radio emission consistent with the interaction of a blastwave with $v_{sh}$~0.1c with a dense environment ($\dot M\sim10^{-3}-10^{-4}\,M_{\odot}yr^{-1}$ for $v_w=1000$ km\s). While these properties exclude Ni-powered transients, our multi-wavelength analysis instead indicates that AT2018cow harbored a "central engine", either a compact object (magnetar or black hole) or an embedded internal shock produced by interaction with a compact, dense circumstellar medium. The engine released $\sim10^{50}-10^{51.5}$ erg over $\sim10^3-10^5$ s and resides within low-mass fast-moving material with equatorial-polar density asymmetry ($M_{ej,fast}\lesssim0.3\,\rm{M_{\odot}}$). Successful SNe from low-mass H-rich stars (like electron-capture SNe) or failed explosions from blue supergiants satisfy these constraints. Intermediate-mass black-holes are disfavored by the large environmental density probed by the radio observations., Comment: 26 pages + tables. Submitted

Vela X is the prototypical example of a pulsar wind nebula whose morphology and detailed structure have been affected by the interaction with the reverse shock of its host supernova remnant. The resulting complex of filamentary structure and mixed-in ejecta embedded in a nebula that is offset from the pulsar provides the best example we have of this middle-age state that characterizes a significant fraction of composite SNRs, and perhaps all of the large-diameter PWNe seen as TeV sources. Here we report on an XMM-Newton Large Project study of Vela X, supplemented by additional Chandra observations. Through broad spectral modeling as well as detailed spectral investigations of discrete emission regions, we confirm previous studies that report evidence for ejecta material within Vela X, and show that equivalent width variations of O VII and O VIII are consistent with temperature maps within the PWN that show low-temperature regions where the projected SNR emission appears to dominate emission from the ejecta. We identify spectral variations in the nonthermal emission, with hard emission being concentrated near the pulsar. We carry out investigations of the Vela X "cocoon" structure and, with hydrodynamical simulations, show that its overall properties are consistent with structures formed in the late-phase evolution of a composite SNR expanding into a surrounding medium with a density gradient, with ejecta material being swept beyond the pulsar and compressed into an elongated structure in the direction opposite the high external density., Comment: 13 pages, 13 figures. Accepted for publication in the Astrophysical Journal

We mapped 12CO J=1-0, 12CO J=2-1, 13CO J=1-0, and 13CO J=2-1 lines toward supernova remnant (SNR) Cassiopeia A with the IRAM 30m telescope. The molecular clouds (MCs) along the line of sight of Cas A do not show optically thin, shock-broadened 12CO lines ($\Delta V \le 7$ km s$^{-1}$ toward Cas A), or high-temperature features from shock heating ($T_k \le 22$ K toward Cas A). Therefore, we suggest that there is no physical evidence to support that the SNR is impacting the molecular gas. All the detected MCs are likely in front of Cas A, as implied by the HCO+ absorption line detected in the same velocity ranges. These MCs contribute H$_2$ column densities of $5\times 10^{21}$ cm$^{-2}$, $5\times 10^{21}$ cm$^{-2}$, and $2\times 10^{21}$ cm$^{-2}$ in the west, south, and center of the SNR, respectively. The 20 K warm gas at $V_{LSR}\sim -47$ km s$^{-1}$ is distributed along a large-scale molecular ridge in the south of Cas A. Part of the gas is projected onto Cas A, providing a foreground H$_2$ mass of $\sim 200 (d/3 kpc)^2$ Msun, consistent with the mass of cold dust (15--20 K; 2--4 Msun) found in front of the SNR. We suggest that the 20 K warm gas is heated by background cosmic rays with an ionization rate of $\zeta({\rm H_2})\sim 2\times 10^{-16}$ s$^{-1}$. The cosmic rays or X-ray emission from Cas A are excluded as the heating sources of the clouds., Comment: 18 pages, 8 figures, 2 tables; Accepted to ApJ

We present early-time Swift and Chandra X-ray data along with late-time optical and near-infrared observations of SN 2013by, a Type IIL supernova (SN) that occurred in the nearby spiral galaxy ESO 138$-$G10 (D $\sim 14.8$ Mpc). Optical and NIR photometry and spectroscopy follow the late-time evolution of the supernova from days +89 to +457 post-maximum brightness. The optical spectra and X-ray light curves are consistent with the picture of a SN having prolonged interaction with circumstellar material (CSM) that accelerates the transition from supernova to supernova remnant (SNR). Specifically, we find SN 2013by's H$\alpha$ profile exhibits significant broadening ($\sim$ 10,000 km s$^{-1}$) on day +457, the likely consequence of high-velocity, H-rich material being excited by a reverse shock. A relatively flat X-ray light curve is observed that cannot be modeled using inverse-Compton scattering processes alone but requires an additional energy source most likely originating from the SN-CSM interaction. In addition, we see the first overtone of CO emission near 2.3 $\mu$m on day +152, signaling the formation of molecules and dust in the SN ejecta and is the first time CO has been detected in a Type IIL supernova. We compare SN 2013by to Type IIP supernovae whose spectra show the rarely observed SN-to-SNR transition in varying degrees and conclude that Type IIL SNe may enter the remnant phase at earlier epochs than their Type IIP counterparts., Comment: Accepted for publication in ApJ. 9 Pages, 10 figures, 3 tables

The progenitor systems of the class of "Ca-rich transients" is a key open issue in time domain astrophysics. These intriguing objects exhibit unusually strong calcium line emissions months after explosion, fall within an intermediate luminosity range, are often found at large projected distances from their host galaxies, and may play a vital role in enriching galaxies and the intergalactic medium. Here we present multi-wavelength observations of iPTF15eqv in NGC 3430, which exhibits a unique combination of properties that bridge those observed in Ca-rich transients and Type Ib/c supernovae. iPTF15eqv has among the highest [Ca II]/[O I] emission line ratios observed to date, yet is more luminous and decays more slowly than other Ca-rich transients. Optical and near-infrared photometry and spectroscopy reveal signatures consistent with the supernova explosion of a < 10 solar mass star that was stripped of its H-rich envelope via binary interaction. Distinct chemical abundances and ejecta kinematics suggest that the core collapse occurred through electron capture processes. Deep limits on possible radio emission made with the Jansky Very Large Array imply a clean environment ($n 100 solar masses). Our results challenge the notion that spectroscopically classified Ca-rich transients only originate from white dwarf progenitor systems, complicate the view that they are all associated with large ejection velocities, and indicate that their chemical abundances may vary widely between events., Comment: 24 pages, 16 figures. Closely matches version published in The Astrophysical Journal

Supernovae shape the interstellar medium, chemically enrich their host galaxies, and generate powerful interstellar shocks that drive future generations of star formation. The shock produced by a supernova event acts as a type of time machine, probing the mass loss history of the progenitor system back to ages of $\sim$ 10 000 years before the explosion, whereas supernova remnants probe a much earlier stage of stellar evolution, interacting with material expelled during the progenitor's much earlier evolution. In this chapter we will review how observations of supernova remnants allow us to infer fundamental properties of the progenitor system. We will provide detailed examples of how bulk characteristics of a remnant, such as its chemical composition and dynamics, allow us to infer properties of the progenitor evolution. In the latter half of this chapter, we will show how this exercise may be extended from individual objects to SNR as classes of objects, and how there are clear bifurcations in the dynamics and spectral characteristics of core collapse and thermonuclear supernova remnants. We will finish the chapter by touching on recent advances in the modeling of massive stars, and the implications for observable properties of supernovae and their remnants.

This is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/mnras/stw2253, Supernova (SN) 2015bh (or SNhunt275) was discovered in NGC 2770 on 2015 February with an absolute magnitude of M$_r$ ~ −13.4 mag, and was initially classified as an SN impostor. Here, we present the photometric and spectroscopic evolution of SN 2015bh from discovery to late phases (~1 yr after). In addition, we inspect archival images of the host galaxy up to ~21 yr before discovery, finding a burst ~1 yr before discovery, and further signatures of stellar instability until late 2014. Later on, the luminosity of the transient slowly increases, and a broad light-curve peak is reached after about three months. We propose that the transient discovered in early 2015 could be a core-collapse SN explosion. The pre-SN luminosity variability history, the long-lasting rise and faintness first light-curve peak suggests that the progenitor was a very massive, unstable and blue star, which exploded as a faint SN because of severe fallback of material. Later on, the object experiences a sudden brightening of 3 mag, which results from the interaction of the SN ejecta with circumstellar material formed through repeated past mass-loss events. Spectroscopic signatures of interaction are however visible at all epochs. A similar chain of events was previously proposed for the similar interacting SN 2009ip.

We present an analysis of archival X-ray observations of the Type IIL supernova SN 1979C. We find that its X-ray luminosity is remarkably constant at (6.5 ± 0.1) × 1038 erg s−1 over a period of 12 years between 1995 and 2007. The high and steady luminosity is considered as possible evidence for a stellar-mass (∼5–10 M⊙) black hole accreting material from either a supernova fallback disk or from a binary companion, or possibly from emission from a central pulsar wind nebula. We find that the bright and steady X-ray light curve is not consistent with either a model for a supernova powered by magnetic braking of a rapidly rotating magnetar, or a model where the blast wave is expanding into a dense circumstellar wind.

According to recent results of Ho & Heinke (2009) and Heinke & Ho (2010), the Cassiopeia A supernova remnant contains a young neutron star which has carbon atmosphere and shows noticeable decline of the effective surface temperature. We report a new (November 2010) Chandra observation which confirms the previously reported decline rate. The decline is naturally explained if neutrons have recently become superfluid (in triplet-state) in the NS core, producing a splash of neutrino emission due to Cooper pair formation (CPF) process that currently accelerates the cooling. This scenario puts stringent constraints on poorly known properties of NS cores: on density dependence of the temperature $T_\mathrm{cn}(\rho)$ for the onset of neutron superfluidity [$T_\mathrm{cn}(\rho)$ should have a wide peak with maximum $\approx (7-9)\times 10^8$ K], on the reduction factor $q$ of CPF process by collective effects in superfluid matter ($q > 0.4$), and on the intensity of neutrino emission before the onset of neutron superfluidity (30--100 times weaker than the standard modified Urca process). This is serious evidence for nucleon superfluidity in NS cores that comes from observations of cooling NSs.

We present an optical spectrum of the energetic Type Ib supernova (SN) 2012au obtained at an unprecedented epoch of 6.2 years after explosion. Forbidden transition emission lines of oxygen and sulfur are detected with expansion velocities of 2300 km/s. The lack of narrow H Balmer lines suggests that interaction with circumstellar material is not a dominant source of the observed late-time emission. We also present a deep Chandra observation that reveals no X-ray emission down to a luminosity of L_X < 2 x 10^{38} erg/s (0.5-10 keV). Our findings are consistent with the notion that SN 2012au is associated with a diverse subset of SNe, including long-duration gamma-ray burst SNe and superluminous SNe, harboring pulsar/magnetar wind nebulae that influence core-collapse explosion dynamics on a wide range of energy scales. We hypothesize that these systems may all evolve into a similar late-time phase dominated by forbidden oxygen transitions, and predict that emission line widths should remain constant or broaden a few per cent per year due to the acceleration of ejecta by the pulsar/magnetar bubble., 9 pages, 4 figures, accepted for publication in ApJL

We model the broadband emission from supernova remnant (SNR) RX J1713.7-3946 including, for the first time, a consistent calculation of thermal X-ray emission together with non-thermal emission in a nonlinear diffusive shock acceleration model. Our model tracks the evolution of the SNR including the plasma ionization state between the forward shock and the contact discontinuity. We use a plasma emissivity code to predict the thermal X-ray emission spectrum assuming the initially cold electrons are heated either by Coulomb collisions with the shock-heated protons (the slowest possible heating), or come into instant equilibration with the protons. For either electron heating model, electrons reach 107 K rapidly and the X-ray line emission near 1 keV is more than 10 times as luminous as the underlying thermal bremsstrahlung continuum. Since recent Suzaku observations show no detectable line emission, this places strong constraints on the unshocked ambient medium density and on the relativistic electron-to-proton ratio. For the uniform circumstellar medium (CSM) models that we consider, the low densities and high relativistic electron-to-proton ratios required to match the Suzaku X-ray observations definitively rule out pion decay as the emission process producing GeV-TeV photons. We show that leptonic models, where inverse-Compton scattering against the cosmic background radiation dominates the GeV-TeV emission, produce better fits to the broadband thermal and non-thermal observations in a uniform CSM.

There is now substantial evidence that the progenitors of some core-collapse supernovae undergo enhanced or extreme mass loss prior to explosion. The imprint of this mass loss is observed in the spectra and dynamics of the expanding blastwave on timescales of days to years after core-collapse, and the effects on the spectral and dynamical evolution may linger long after the supernova has evolved into the remnant stage. In this paper, we present for the first time, largely self-consistent end-to-end simulations for the evolution of a massive star from the pre-main sequence, up to and through core collapse, and into the remnant phase. We present three models and compare and contrast how the progenitor mass loss history impacts the dynamics and spectral evolution of the supernovae and supernova remnants. We study a model which only includes steady mass loss, a model with enhanced mass loss over a period of $\sim$ 5000 years prior to core-collapse, and a model with extreme mass loss over a period of $\sim$ 500 years prior to core collapse. The models are not meant to address any particular supernova or supernova remnant, but rather to highlight the important role that the progenitor evolution plays in the observable qualities of supernovae and supernova remnants. Through comparisons of these three different progenitor evolution scenarios, we find that the mass loss in late stages (during and after core carbon burning) can have a profound impact on the dynamics and spectral evolution of the supernova remnant centuries after core-collapse., Comment: 18 pages, 11 figures; submitted to the Astrophysical Journal

We present results of semi-analytic calculations which show clear evidence for changes in the non-equilibrium ionization behind a supernova remnant forward shock undergoing efficient diffusive shock acceleration (DSA). The efficient acceleration of particles (i.e., cosmic rays) lowers the shock temperature and raises the density of the shocked gas, thus altering the ionization state of the plasma in comparison to the test particle approximation where cosmic rays gain an insignificant fraction of the shock energy. The differences between the test particle and efficient acceleration cases are substantial and occur for both slow and fast temperature equilibration rates: in cases of higher acceleration efficiency, particular ion states are more populated at lower electron temperatures. We also present results which show that, in the efficient shock acceleration case, higher ionization fractions are reached noticeably closer to the shock front than in the test-particle case, clearly indicating that DSA may enhance thermal X-ray production. We attribute this to the higher postshock densities which lead to faster electron temperature equilibration and higher ionization rates. These spatial differences should be resolvable with current and future X-ray missions, and can be used as diagnostics in estimating the acceleration efficiency in cosmic-ray modified shocks., Comment: 13 pages, 7 figures, accepted for publication by the Astrophysical Journal

We present new observational results obtained for the Galactic non-thermal radio source G328.4+0.2 to determine both if this source is a pulsar wind nebula or supernova remnant, and in either case, the physical properties of this source. Using X-ray data obtained by XMM, we confirm that the X-ray emission from this source is heavily absorbed and has a spectrum best fit by a power law model of photon index=2 with no evidence for a thermal component, the X-ray emission from G328.4+0.2 comes from a region significantly smaller than the radio emission, and that the X-ray and radio emission are significantly offset from each other. We also present the results of a new high resolution (7 arcseconds) 1.4 GHz image of G328.4+0.2 obtained using the Australia Telescope Compact Array, and a deep search for radio pulsations using the Parkes Radio Telescope. We find that the radio emission has a flat spectrum, though some areas along the eastern edge of G328.4+0.2 have a steeper radio spectral index of ~-0.3. Additionally, we obtain a luminosity limit of the central pulsar of L_{1400} < 30 mJy kpc^2, assuming a distance of 17 kpc. In light of these observational results, we test if G328.4+0.2 is a pulsar wind nebula (PWN) or a large PWN inside a supernova remnant (SNR) using a simple hydrodynamic model for the evolution of a PWN inside a SNR. As a result of this analysis, we conclude that G328.4+0.2 is a young (< 10000 years old) pulsar wind nebula formed by a low magnetic field (10^51 ergs), low ejecta mass (M < 5 Solar Masses) supernova explosion which occurred in a low density (n~0.03 cm^{-3}) environment., 27 pages, 11 figures, accepted for publication in The Astrophysical Journal

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 optical observations of supernova SN 2014C, which underwent an unprecedented slow metamorphosis from H-poor type Ib to H-rich type IIn over the course of one year. The observed spectroscopic evolution is consistent with the supernova having exploded in a cavity before encountering a massive shell of the progenitor star's stripped hydrogen envelope. Possible origins for the circumstellar shell include a brief Wolf-Rayet fast wind phase that overtook a slower red supergiant wind, eruptive ejection, or confinement of circumstellar material by external influences of neighboring stars. An extended high velocity Halpha absorption feature seen in near-maximum light spectra implies that the progenitor star was not completely stripped of hydrogen at the time of core collapse. Archival pre-explosion Subaru Telescope Suprime-Cam and Hubble Space Telescope Wide Field Planetary Camera 2 images of the region obtained in 2009 show a coincident source that is most likely a compact massive star cluster in NGC 7331 that hosted the progenitor system. By comparing the emission properties of the source with stellar population models that incorporate interacting binary stars we estimate the age of the host cluster to be 30 - 300 Myr, and favor ages closer to 30 Myr in light of relatively strong Halpha emission. SN 2014C is the best-observed member of a class of core-collapse supernovae that fill the gap between events that interact strongly with dense, nearby environments immediately after explosion and those that never show signs of interaction. Better understanding of the frequency and nature of this intermediate population can contribute valuable information about the poorly understood final stages of stellar evolution., Comment: 13 pages, 7 figures, accepted to ApJ

We present optical observations and 2D hydrodynamic modeling of an isolated shocked ISM cloud. H$\alpha$ images taken in 1992.6 and 2003.7 of a small optical emission cloud along the southwestern limb of the Cygnus Loop were used to measure positional displacements of $\sim$ $0 \farcs 1$ yr$^{-1}$ for surrounding Balmer dominated emission filaments and $0\farcs025 - \farcs055$ yr$^{-1}$ for internal cloud emission features. These measurements imply transverse velocities of $\simeq$ 250 km s$^{-1}$ and $\simeq$ 80 -- 140 km s$^{-1}$ for ambient ISM and internal cloud shocks respectively. The complex shock structure visible within the cloud indicates that the cloud's internal density distribution is two phased: a smoothly varying background density which is populated by higher density clumps. We present model results for a shock interacting with a non-uniform ISM cloud. We find that this cloud can be well modeled by a smoothly varying power law core surrounded by a low density envelope with a Lorentzian profile. The lack of sharp density gradients in such a model inhibits the growth of Kelvin-Helmholtz instabilities, consistent with the cloud's appearance. Our model results also suggest that cloud clumps have densities $\sim$ 10 times the ambient ISM density and account for $\sim$ 30% of the total cloud volume. Moreover, the observed spacing of internal cloud shocks and model simulations indicate that the distance between clumps is $\sim$ 4 clump radii., Comment: To be published in ApJ

An analysis of a 29 ks Chandra ACIS-S observation of the young, Cassiopeia A-like supernova remnant in the irregular galaxy NGC 4449 is presented. The observed 0.5-2.1 keV spectrum reveals the likely presence of several emission lines, including O VIII at 0.65 and 0.77 keV, Ne X at 1.05 keV, Mg XI at 1.5 keV, and Si XIII at 1.85 keV. From the observed spectrum, we derive a NH = 1 × 1021 cm-2 and an X-ray temperature of T ≈ 9 × 106 K. A nonequilibrium ionization fit to the spectrum suggests an overabundance of oxygen around 20 times solar, consistent with the remnant's UV and optical emission-line properties. We discuss the remnant's approximate X-ray-derived elemental abundances and compare its X-ray spectrum and luminosity with other oxygen-rich remnants.

Spatially resolved ROSAT X-ray and ground-based optical data for the southwestern region of the Cygnus Loop SNR reveal in unprecedented detail the very early stages of a blast wave interaction with an isolated interstellar cloud. Numerous internal cloud shock fronts near the upstream flow and along the cloud edges are visible optically as sharp filaments of enhanced H-alpha emission. Faint X-ray emission is seen along a line of Balmer-dominated shock filaments north and south of the cloud with an estimated X-ray gas temperature of 1.2 x 10^6 K (0.11 keV) corresponding to a shock velocity of 290 km s^-1. The main cloud body itself exhibits little or no X-ray flux. Instead, X-ray emission is confined along the northern and southernmost cloud edges, with the emission brightest in the downstream regions farthest from the shock front's current position. We estimate an interaction age of around 1200 yr based on the observed shock/cloud morphology. Overall, the optical and X-ray properties of this shocked ISM cloud show many of the principal features predicted for a young SNR shock -- ISM cloud interaction. In particular, one sees shocklet formation and diffraction inside the inhomogenous cloud along with partial main blast wave engulfment. However, several significant differences from model predictions are also present including no evidence for turbulence along cloud edges, diffuse rather than filamentary [O III] emission within the main body of the cloud, unusually strong downstream [S II] emission in the postshock cloud regions, and confinement of X-ray emission to the cloud's outer boundaries.

The cooling rate of young neutron stars gives direct insight into their internal makeup. Using Chandra observations of the 330-year-old Cassiopeia A supernova remnant, we find that the temperature of the youngest-known neutron star in the Galaxy has declined by 4% over the last 10 years. The decline is explained naturally by superconductivity and superfluidity of the protons and neutrons in the stellar core. The protons became superconducting early in the life of the star and suppressed the early cooling rate; the neutron star thus remained hot before the (recent) onset of neutron superfluidity. Once the neutrons became superfluid, the Cooper pair-formation process produced a splash of neutrino emission which accelerated the cooling and resulted in the observed rapid temperature decline. This is the first time a young neutron star has been seen to cool in real time, and is the first direct evidence, from cooling observations, of superfluidity and superconductivity in the core of neutron stars.

Broadband optical and narrowband Si XIII X-ray images of the young Galactic supernova remnant Cas A obtained over several decades are used to investigate spatial and temporal correlations on both large and small scales. The data consist of optical and near infrared ground-based and Hubble Space Telescope images taken between 1951 and 2011, and X-ray images from Einstein, ROSAT, and Chandra taken between 1979 and 2013. We find weak spatial correlations between the remnant's emission features on large scales, but several cases of good optical/X-ray correlations on small scales for features which have brightened due to recent interaction with the reverse shock. We also find instances where: (i) a time delay is observed between the appearance of a feature's optical and X-ray emissions, (ii) displacements of several arcseconds between a feature's X-ray and optical emission peaks and, (iii) regions showing no corresponding X-ray or optical emissions. To explain this behavior, we propose a inhomogeneous model for Cas A's ejecta consisting of small, dense optically emitting knots (n ~ 10^(2-3)/cm^(3)) and a much lower density (n ~ 0.1 - 1/cm^(3)) diffuse X-ray emitting component often spatially associated with optical emission knots. The X-ray emitting component is sometimes linked to optical clumps through shock induced mass ablation generating trailing material leading to spatially offset X-ray/optical emissions. A range of ejecta densities can also explain the observed X-ray/optical time delays since the remnant's 5000 km/s reverse shock heats dense ejecta clumps to temperatures around 3x10^4 K relatively quickly which then become optically bright while more diffuse ejecta become X-ray bright on longer timescales. Highly inhomogeneous ejecta as proposed here for Cas A may help explain some of the X-ray/opticalfeatures seen in other young core collapse SN remnants., Comment: 31 pages, 21 figures. Submitted to the Astrophysical Journal. Please contact the corresponding author for higher resolution postscript versions of the figures (dpatnaude@cfa.harvard.edu)

Although collisionless shocks are ubiquitous in astrophysics, certain key aspects of them are not well understood. In particular, the process known as collisionless electron heating, whereby electrons are rapidly energized at the shock front, is one of the main open issues in shock physics. Here we present the first clear evidence for efficient collisionless electron heating at the reverse shock of Tycho's supernova remnant (SNR), revealed by Fe-K diagnostics using high-quality X-ray data obtained by the Suzaku satellite. We detect K-beta (3p->1s) fluorescence emission from low-ionization Fe ejecta excited by energetic thermal electrons at the reverse shock front, which peaks at a smaller radius than Fe K-alpha (2p->1s) emission dominated by a relatively highly-ionized component. Comparison with our hydrodynamical simulations implies instantaneous electron heating to a temperature 1000 times higher than expected from Coulomb collisions alone. The unique environment of the reverse shock, which is propagating with a high Mach number into rarefied ejecta with a low magnetic field strength, puts strong constraints on the physical mechanism responsible for this heating, and favors a cross-shock potential created by charge deflection at the shock front. Our sensitive observation also reveals that the reverse shock radius of this SNR is about 10% smaller than the previous measurement using the Fe K-alpha morphology from the Chandra observations. Since strong Fe K-beta fluorescence is expected only from low-ionization plasma where Fe ions still have many 3p electrons, this feature is key to diagnosing the plasma state and distribution of the immediate postshock ejecta in a young SNR., 7 pages, 9 figures, resubmitted to ApJ with minor changes following the referee report

The thermal evolution of young neutron stars (NSs) reflects the neutrino emission properties of their cores. Heinke et al. (2010) measured a 3.6+/-0.6% decay in the surface temperature of the Cassiopeia A (Cas A) NS between 2000 and 2009, using archival data from the Chandra X-ray Observatory ACIS-S detector in Graded mode. Page et al. (2011) and Shternin et al. (2011) attributed this decay to enhanced neutrino emission from a superfluid neutron transition in the core. Here we test this decline, combining analysis of the Cas A NS using all Chandra X-ray detectors and modes (HRC-S, HRC-I, ACIS-I, ACIS-S in Faint mode, and ACIS-S in Graded mode) and adding a 2012 May ACIS-S Graded mode observation, using the most current calibrations (CALDB 4.5.5.1). We measure the temperature changes from each detector separately and test for systematic effects due to the nearby filaments of the supernova remnant. We find a 0.92%-2.0% decay over 10 years in the effective temperature, inferred from HRC-S data, depending on the choice of source and background extraction regions, with a best-fit decay of 1.0+/-0.7%. In comparison, the ACIS-S Graded data indicate a temperature decay of 3.1%-5.0% over 10 years, with a best-fit decay of 3.5+/-0.4%. Shallower observations using the other detectors yield temperature decays of 2.6+/-1.9% (ACIS-I), 2.1+/-1.0% (HRC-I), and 2.1+/-1.9% (ACIS-S Faint mode) over 10 years. Our best estimate indicates a decline of 2.9+/-0.9 (stat) +1.6/-0.3 (sys) % over 10 years. The complexity of the bright and varying supernova remnant background makes a definitive interpretation of archival Cas A Chandra observations difficult. A temperature decline of 1-3.5% over 10 years would indicate extraordinarily fast cooling of the NS that can be regulated by superfluidity of nucleons in the stellar core., Comment: 9 pages, 8 figures, Accepted in ApJ

For supernova powered by the conversion of kinetic energy into radiation due to the interactions of the ejecta with a dense circumstellar shell, we show that there could be X-ray analogues of optically super-luminous SNe with comparable luminosities and energetics. We consider X-ray emission from the forward shock of SNe ejecta colliding into an optically-thin CSM shell, derive simple expressions for the X-ray luminosity as a function of the circumstellar shell characteristics, and discuss the different regimes in which the shock will be radiative or adiabatic, and whether the emission will be dominated by free-free radiation or line-cooling. We find that even with normal supernova explosion energies of 10^51 erg, there exists CSM shell configurations that can liberate a large fraction of the explosion energy in X-rays, producing unabsorbed X-ray luminosities approaching 10^44 erg/s events lasting a few months, or even 10^45 erg/s flashes lasting days. Although the large column density of the circumstellar shell can absorb most of the flux from the initial shock, the most luminous events produce hard X-rays that are less susceptible to photoelectric absorption, and can counteract such losses by completely ionizing the intervening material. Regardless, once the shock traverses the entire circumstellar shell, the full luminosity could be available to observers., Submitted to MNRAS. 12 pages, 4 figures

Based largely on energy budget considerations and the observed cosmic-ray (CR) ionic composition, supernova remnant (SNR) blast waves are the most likely sources of CR ions with energies at least up to the "knee" near 3 PeV. Shocks in young shell-type TeV-bright SNRs are surely producing TeV particles, but the emission could be dominated by ions producing neutral pion-decay emission or electrons producing inverse-Compton gamma-rays. Unambiguously identifying the GeV-TeV emission process in a particular SNR will not only help pin down the origin of CRs, it will add significantly to our understanding of the diffusive shock acceleration (DSA) mechanism and improve our understanding of supernovae and the impact SNRs have on the circumstellar medium. In this study, we investigate the Vela Jr. SNR, an example of TeV-bright non-thermal SNRs. We perform hydrodynamic simulations coupled with non-linear DSA and non-equilibrium ionization near the forward shock (FS) to confront currently available multi-wavelength data. We find, with an analysis similar to that used earlier for SNR RX J1713.7-3946, that self-consistently modeling the thermal X-ray line emission with the non-thermal continuum in our one-dimensional model strongly constrains the fitting parameters, and this leads convincingly to a leptonic origin for the GeV-TeV emission for Vela Jr. This conclusion is further supported by applying additional constraints from observation, including the radial brightness profiles of the SNR shell in TeV gamma-rays, and the spatial variation of the X-ray synchrotron spectral index. We will discuss implications of our models on future observations by the next-generation telescopes., Comment: 12 pages, 10 figures, to appear at the Astrophysical Journal

CTB 109 (G109.1-1.0) is a Galactic supernova remnant (SNR) with a hemispherical shell morphology in X-rays and in the radio band. In this work we report the detection of {\gamma}-ray emission coincident with CTB 109, using 37 months of data from the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. We study the broadband characteristics of the remnant using a model that includes hydrodynamics, efficient cosmic ray acceleration, nonthermal emission and a self-consistent calculation of the X-ray thermal emission. We find that the observations can be successfully fit with two distinct parameter sets, one where the {\gamma}-ray emission is produced primarily by leptons accelerated at the SNR forward shock and the other where {\gamma}-rays produced by forward shock accelerated cosmic-ray ions dominate the high-energy emission. Consideration of thermal X-ray emission introduces a novel element to the broadband fitting process, and while it does not rule out either the leptonic or the hadronic scenarios, it constrains the parameter sets required by the model to fit the observations. Moreover, the model which best fits the thermal and nonthermal emission observations is an intermediate case, where both radiation from accelerated electrons and hadrons contribute almost equally to the {\gamma}-ray flux observed., Comment: 10 pages, 6 figures. Accepted for publication in ApJ

It is now well established that Kepler's supernova remnant is the result of a Type Ia explosion. With an age of 407 years, and an angular diameter of ~ 4', Kepler is estimated to be between 3.0 and 7.0 kpc distant. Unlike other Galactic Type Ia supernova remnants such as Tycho and SN 1006, and SNR 0509-67.5 in the Large Magellanic Cloud, Kepler shows evidence for a strong circumstellar interaction. A bowshock structure in the north is thought to originate from the motion of a mass-losing system through the interstellar medium prior to the supernova. We present results of hydrodynamical and spectral modeling aimed at constraining the circumstellar environment of the system and the amount of 56Ni produced in the explosion. Using models that contain either 0.3 M_sun (subenergetic) or 1 M_sun (energetic) of 56Ni, we simulate the interaction between supernova Ia ejecta and various circumstellar density models. Based on dynamical considerations alone, we find that the subenergetic models favor a distance to the SNR of < 6.4 kpc, while the model that produces 1 M_sun of 56Ni requires a distance to the SNR of > 7 kpc. The X-ray spectrum is consistent with an explosion that produced ~ 1 M_sun of 56Ni, ruling out the subenergetic models, and suggesting that Kepler's SNR was a SN 1991T-like event. Additionally, the X-ray spectrum rules out a pure 1/r^2 wind profile expected from isotropic mass loss up to the time of the supernova. Introducing a small cavity around the progenitor system results in modeled X-ray spectra that are consistent with the observed spectrum. If a wind shaped circumstellar environment is necessary to explain the dynamics and X-ray emission from the shocked ejecta in Kepler's SNR, then we require that the distance to the remnant be greater than 7 kpc., 12 pages, 6 figures. Accepted for publication in the Astrophysical Journal

Although the connection of the Chinese "guest" star of 393 AD with the Galactic supernova remnant RX J1713.7-3946 (G347.3-0.5) made by Wang et al. in 1997 is consistent with the remnant's relatively young properties and the guest star's projected position within the `tail' of the constellation Scorpius, there are difficulties with such an association. The brief Chinese texts concerning the 393 AD guest star make no comment about its apparent brightness stating only that it disappeared after 8 months. However, at the remnant's current estimated 1 - 1.3 kpc distance and A_v ~ 3 mag, its supernova should have been a visually bright object at maximum light (-3.5 to -5.0 mag) and would have remained visible for over a year. The peak brightness ~ 0 magnitude adopted by Wang et al. and others would require the RX J1713.7-3946 supernova to have been a very subluminous supernova event similar to or fainter than CCSNe like SN 2005cs. We also note problems connecting SN 393 with a European record in which the Roman poet Claudian describes a visually brilliant star in the heavens around 393 AD that could be readily seen even in midday. Although several authors have suggested this account may be a reference to the Chinese supernova of 393, Scorpius would not be visible near midday in March when the Chinese first reported the 393 guest star. We review both the Chinese and Roman accounts and calculate probable visual brightnesses for a range of supernova subtypes and conclude that neither the Chinese nor the Roman descriptions are easily reconciled with an expected RX J1713.7-3946 supernova brightness and duration., Comment: 7 pages includes 2 tables. Accepted for publication in AJ. Revised text regarding subluminous SNe

The Extreme Physics Explorer (EPE) is a concept timing/spectroscopy mission that would use micro-channel plate optics (MCPO) to provide 4 square meters effective area focused to ~1 arc-min onto an X-ray calorimeter. We describe science drivers for such a mission, possible designs for the large area MCPO needed for EPE, and the challenges of the large area MCPO design., Comment: 12 pages, 9 figures, to appear in proceedings of SPIE V8147, "Optics for EUV, X-ray, and Gamma-Ray Astronomy V"

We present a spherically symmetric, core-collapse model of SNR RX J1713.7-3946 that includes a hydrodynamic simulation of the remnant evolution coupled to the efficient production of cosmic rays (CRs) by nonlinear diffusive shock acceleration (DSA). High-energy CRs that escape from the forward shock (FS) are propagated in surrounding dense material that simulates either a swept-up, pre-supernova shell or a nearby molecular cloud. The continuum emission from trapped and escaping CRs, along with the thermal X-ray emission from the shocked heated ISM behind the FS, integrated over the remnant, is compared against broadband observations. Our results show conclusively that, overall, the GeV-TeV emission is dominated by inverse-Compton from CR electrons if the supernova is isolated regardless of its type, i.e., not interacting with a >>100 Msun shell or cloud. If the SNR is interacting with a much larger mass >10^4 Msun, pion-decay from the escaping CRs may dominate the TeV emission, although a precise fit at high energy will depend on the still uncertain details of how the highest energy CRs are accelerated by, and escape from, the FS. Based on morphological and other constraints, we consider the 10^4 Msun pion-decay scenario highly unlikely for SNR RX J1713.7-3946 regardless of the details of CR escape. Importantly, even though CR electrons dominate the GeV-TeV emission, the efficient production of CR ions is an essential part of our leptonic model., 11 pages with 7 figures. Submitted to ApJ

We investigate the effects of the efficient production of cosmic rays on the evolution of supernova remnants (SNRs) in the adiabatic Sedov-Taylor phase. We model the SNR by coupling the hydrodynamic evolution with nonlinear diffusive shock acceleration (DSA), and track self-consistently the ionization state of the shock-heated plasma. Using a plasma emissivity code and the results of the model, we predict the thermal X-ray emission and combine it with the non-thermal component in order to obtain the complete spectrum in this energy range. Hence, we study how the interpretation of thermal X-ray observations is affected by the efficiency of the DSA process, and find that, compared to test particle cases, the efficient DSA example yields a smaller shock radius and speed, a larger compression ratio, and lower intensity X-ray thermal emission. We also find that a model where the shock is not assumed to produce CRs can fit the X-ray observational properties of an example with efficient particle acceleration, with a different set of input parameters, and in particular a much lower explosion energy. Additionally, we model the broadband non-thermal emission, and investigate what signatures result from the acceleration of particles., Comment: 8 Pages, 9 figures. Accepted for publication in ApJ

We present a grid of nonequilibrium ionization models for the X-ray spectra from supernova remnants undergoing efficient diffusive shock acceleration. The calculation follows the hydrodynamics of the blast wave as well as the time-dependent ionization of the plasma behind the shock. The ionization state is passed to a plasma emissivity code to compute the thermal X-ray emission, which is combined with the emission from nonthermal synchrotron emission to produce a self-consistent model for the thermal and nonthermal emission from cosmic-ray dominated shocks. We show how plasma diagnostics such as the G'-ratio of He-like ions, defined as the ratio of the sum of the intercombination, forbidden, and satellite lines to the resonance line, can vary with acceleration efficiency, and discuss how the thermal X-ray emission, when the time-dependent ionization is not calculated self-consistently with the hydrodynamics, can differ from the thermal X-ray emission from models which do account for the hydrodynamics. Finally we compare the thermal X-ray emission from models which show moderate acceleration (~ 35%) to the thermal X-ray emission from test-particle models., 17 pages, 12 figures. accepted for publication in the Astrophysical Journal

The International X-ray Observatory (IXO) has a top level requirement that the observing efficiency be 85%. This is a challenging requirement, given that the observing efficiencies for CXO and XMM-Newton are between 60% and 70%. However, the L2 orbit for IXO means that it will not be subject to the earth block/radiation zone effects that are seen for CXO and XMM-Newton. Outside of these effects the efficiencies for CXO and XMM-Newton do approach 85%, so this requirement appears achievable for IXO. In this paper we itemize the effects which impact the observing efficiency, in order to guide the design of the observatory. Meeting the 85% requirement should be possible but will require careful attention to detail.