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We present HST/WFC3 multiband imagery of N103B, the remnant of a Type Ia supernova in the Large Magellanic Cloud, as well as HST/COS ultraviolet spectroscopy of the brightest radiatively shocked region. The images show a wide range of morphology and relative emission-line intensities, from smooth Balmer-line dominated collisionless shocks due to the primary blast wave, to clumpy radiative shock filaments due to secondary shocks in density enhancements. The COS data show strong FUV line emission despite a moderately high extinction along this line of sight. We use the COS data with previous optical spectra to constrain the shock conditions and refine the abundance analysis, finding abundances typical of the local interstellar medium within the uncertainties. Under an assumption that the material being shocked was shed from the pre-supernova system, this finding places constraints on any significant enrichment in that material, and thus on the non-degenerate star in what was presumably a single-degenerate Type Ia supernova., 25 pages, 6 figures, 2 tables, accepted for Astrophysical Journal, August 27, 2020

Some tidal disruption events (TDEs) exhibit blueshifted broad absorption lines (BALs) in their rest-frame ultraviolet (UV) spectra, while others display broad emission lines (BELs). Similar phenomenology is observed in quasars and accreting white dwarfs, where it can be interpreted as an orientation effect associated with line formation in an accretion disc wind. We propose and explore a similar unification scheme for TDEs. We present synthetic UV spectra for disc and wind-hosting TDEs, produced by a state-of-the-art Monte Carlo ionization and radiative transfer code. Our models cover a wide range of disc wind geometries and kinematics. Such winds naturally reproduce both BALs and BELs. In general, sightlines looking into the wind cone preferentially produce BALs, while other orientations preferentially produce BELs. We also study the effect of wind clumping and CNO-processed abundances on the observed spectra. Clumpy winds tend to produce stronger UV emission and absorption lines, because clumping increases both the emission measure and the abundances of the relevant ionic species, the latter by reducing the ionization state of the outflow. The main effect of adopting CNO-processed abundances is a weakening of C iv 1550 Å and an enhancement of N v 1240 Å in the spectra. We conclude that line formation in an accretion disc wind is a promising mechanism for explaining the diverse UV spectra of TDEs. If this is correct, the relative number of BAL and BEL TDEs can be used to estimate the covering factor of the outflow. The models in this work are publicly available online and upon request.

NGC6946 is a high star formation rate face-on spiral galaxy that has hosted ten supernovae since 1917. Not surprisingly, a large number of supernova remnants and candidates have been identified either as optical nebulae with high [S II]:H$��$ line ratios (147) or as compact non-thermal radio sources (35). However, there are only seven overlaps between these two samples. Here, we apply [Fe II] 1.644 $��$m emission as a new diagnostic to search for supernova remnants in an attempt to resolve this discrepancy. [Fe II] is expected to be relatively strong in the radiative shocks of supernova remnants and almost absent in HII regions. It is less susceptible to the effects of absorption along the line of sight than the optical lines normally used to identify remnants. Using data from the WFC3 camera on HST}, we identify 132 [Fe II] emission nebulae in NGC6946 as likely supernova remnants. Of these, 54 align with previously known optical supernova remnants. The remaining 78 objects are new; of these 44 are visible in new HST imagery in H$��$ and [S II]. This brings the total number of supernova remnant candidates (from optical and/or IR data) in NGC6946 to 225. A total of 14 coincidences with radio supernova remnant candidates (out of 30 in our search area) are found in this expanded list. The identification of so many new remnant candidates validates the use of [Fe II] imagery for finding remnants, and suggests that previous remnant searches in other galaxies may be far from complete., 43 pages, 9 figures, 2 tables, accepted for publication in ApJ

Supernova remnants (SNRs) arise from the interaction between the ejecta of a supernova (SN) explosion and the surrounding circumstellar and interstellar medium. Some SNRs, mostly nearby SNRs, can be studied in great detail. However, to understand SNRs as a whole, large samples of SNRs must be assembled and studied. Here, we describe the radio, optical, and X-ray techniques which have been used to identify and characterize almost 300 Galactic SNRs and more than 1200 extragalactic SNRs. We then discuss which types of SNRs are being found and which are not. We examine the degree to which the luminosity functions, surface-brightness distributions and multi-wavelength comparisons of the samples can be interpreted to determine the class properties of SNRs and describe efforts to establish the type of SN explosion associated with a SNR. We conclude that in order to better understand the class properties of SNRs, it is more important to study (and obtain additional data on) the SNRs in galaxies with extant samples at multiple wavelength bands than it is to obtain samples of SNRs in other galaxies, Final 2016 draft of a chapter in "Handbook of Supernovae" edited by Athem W. Alsabti and Paul Murdin. Final version available at https://doi.org/10.1007/978-3-319-20794-0_90-1

The Crab nebula originated from a core-collapse supernova (SN) explosion observed in 1054 A.D. When viewed as a supernova remnant (SNR), it has an anomalously low observed ejecta mass and kinetic energy for an Fe-core collapse SN. Intensive searches were made for a massive shell that solves this discrepancy, but none has been detected. An alternative idea is that the SN1054 is an electron-capture (EC) explosion with a lower explosion energy by an order of magnitude than Fe-core collapse SNe. In the X-rays, imaging searches were performed for the plasma emission from the shell in the Crab outskirts to set a stringent upper limit to the X-ray emitting mass. However, the extreme brightness of the source hampers access to its vicinity. We thus employed spectroscopic technique using the X-ray micro-calorimeter onboard the Hitomi satellite. By exploiting its superb energy resolution, we set an upper limit for emission or absorption features from yet undetected thermal plasma in the 2-12 keV range. We also re-evaluated the existing Chandra and XMM-Newton data. By assembling these results, a new upper limit was obtained for the X-ray plasma mass of

The origin of the narrow Fe-K�� fluorescence line at 6.4 keV from active galactic nuclei has long been under debate; some of the possible sites are the outer accretion disk, the broad line region, a molecular torus, or interstellar/intracluster media. In February-March 2016, we performed the first X-ray microcalorimeter spectroscopy with the Soft X-ray Spectrometer (SXS) onboard the Hitomi satellite of the Fanaroff-Riley type I radio galaxy NGC 1275 at the center of the Perseus cluster of galaxies. With the high energy resolution of ~5 eV at 6 keV achieved by Hitomi/SXS, we detected the Fe-K�� line with ~5.4 �� significance. The velocity width is constrained to be 500-1600 km s$^{-1}$ (FWHM for Gaussian models) at 90% confidence. The SXS also constrains the continuum level from the NGC 1275 nucleus up to ~20 keV, giving an equivalent width ~20 eV of the 6.4 keV line. Because the velocity width is narrower than that of broad H�� line of ~2750 km s$^{-1}$, we can exclude a large contribution to the line flux from the accretion disk and the broad line region. Furthermore, we performed pixel map analyses on the Hitomi/SXS data and image analyses on the Chandra archival data, and revealed that the Fe-K�� line comes from a region within ~1.6 kpc from the NGC 1275 core, where an active galactic nucleus emission dominates, rather than that from intracluster media. Therefore, we suggest that the source of the Fe-K�� line from NGC 1275 is likely a low-covering fraction molecular torus or a rotating molecular disk which probably extends from a pc to hundreds pc scale in the active galactic nucleus system., 20 pages, 8 figures, 6 tables, accepted for publication in PASJ

High-resolution X-ray spectroscopy with Hitomi was expected to resolve the origin of the faint unidentified E=3.5 keV emission line reported in several low-resolution studies of various massive systems, such as galaxies and clusters, including the Perseus cluster. We have analyzed the Hitomi first-light observation of the Perseus cluster. The emission line expected for Perseus based on the XMM-Newton signal from the large cluster sample under the dark matter decay scenario is too faint to be detectable in the Hitomi data. However, the previously reported 3.5 keV flux from Perseus was anomalously high compared to the sample-based prediction. We find no unidentified line at the reported high flux level. Taking into account the XMM measurement uncertainties for this region, the inconsistency with Hitomi is at a 99% significance for a broad dark-matter line and at 99.7% for a narrow line from the gas. We do not find anomalously high fluxes of the nearby faint K line or the Ar satellite line that were proposed as explanations for the earlier 3.5 keV detections. We do find a hint of a broad excess near the energies of high-n transitions of Sxvi (E=3.44 keV rest-frame) -- a possible signature of charge exchange in the molecular nebula and another proposed explanation for the unidentified line. While its energy is consistent with XMM pn detections, it is unlikely to explain the MOS signal. A confirmation of this interesting feature has to wait for a more sensitive observation with a future calorimeter experiment., Discussion of systematics significantly expanded. 9 pages, 5 figures; ApJ Lett. in press

Several young supernova remnants (SNRs), including SN1006, emit synchrotron X-rays in narrow filaments, hereafter thin rims, along their periphery. The widths of these rims imply 50 to 100 $\mu$G fields in the region immediately behind the shock, far larger than expected for the interstellar medium compressed by unmodified shocks, assuming electron radiative losses limit rim widths. However, magnetic-field damping could also produce thin rims. Here we review the literature on rim width calculations, summarizing the case for magnetic-field amplification. We extend these calculations to include an arbitrary power-law dependence of the diffusion coefficient on energy, $D \propto E^{\mu}$. Loss-limited rim widths should shrink with increasing photon energy, while magnetic-damping models predict widths almost independent of photon energy. We use these results to analyze Chandra observations of SN 1006, in particular the southwest limb. We parameterize the full widths at half maximum (FWHM) in terms of energy as FWHM $\propto E^{m_E}_{\gamma}$. Filament widths in SN1006 decrease with energy; $m_E \sim -0.3$ to $-0.8$, implying magnetic field amplification by factors of 10 to 50, above the factor of 4 expected in strong unmodified shocks. For SN 1006, the rapid shrinkage rules out magnetic damping models. It also favors short mean free paths (small diffusion coefficients) and strong dependence of $D$ on energy ($\mu \ge 1$)., Comment: Accepted by ApJ, 49 pages, 10 figures

BB Dor was observed during its low state in 2009. Signatures of both binary components are revealed in the average optical spectrum; no signature of accretion is observed. Narrow emission lines of Hα, Hei and NaD, as well as TiO absorption troughs, trace the motion of the irradiated secondary star. We detect two additional components in the Hα emission line that share many characteristics of similar 'satellite' lines observed in the low state of magnetic cataclysmic variables of AM Her type. It is the first time such emission components are detected for an SW Sex star. © 2012 The Authors Monthly Notices of the Royal Astronomical Society © 2012 RAS.

SDSS J1507+52 is an eclipsing cataclysmic variable consisting of a cool, non-radially pulsating white dwarf and an unusually small sub-stellar secondary. The system has a high space velocity and a very short orbital period of about 67 minutes, well below the usual minimum period for CVs. To explain the existence of this peculiar system, two theories have been proposed. One suggests that SDSS J1507+52 was formed from a detached white-dwarf/brown-dwarf binary. The other theory proposes that the system is a member of the Galactic halo-population. Here, we present ultraviolet spectroscopy of SDSS J1507+52 obtained with the Hubble Space Telescope with the aim of distinguishing between these two theories. The UV flux of the system is dominated by emission from the accreting white dwarf. Fits to model stellar atmospheres yield physical parameter estimates of T(eff) = 14200 \pm 500 K, log(g)=8.2 \pm 0.3, vsin(i)=180 \pm 20 kms-1 and [Fe/H]=-1.2 \pm 0.2. These fits suggest a distance towards SDSS J1507+52 of d = 250 \pm 50 pc. The quoted uncertainties include systematic errors associated with the adopted fitting windows and interstellar reddening. Assuming that there is no contribution to the UV flux from a hot, optically thick boundary layer, we find a T(eff) much higher than previously estimated from eclipse analysis. The strongly sub-solar metallicity we infer for SDSS J1507+52 is consistent with that of halo stars at the same space velocity. We therefore conclude that SDSS J1507+52 is a member of the Galactic halo.

We present results from mid-IR spectroscopic observations of two young supernova remnants (SNRs) in the Large Magellanic Cloud (LMC) done with the {\it Spitzer Space Telescope}. We imaged SNRs B0509-67.5 and B0519-69.0 with {\it Spitzer} in 2005, and follow-up spectroscopy presented here confirms the presence of warm, shock heated dust, with no lines present in the spectrum. We use model fits to {\it Spitzer} IRS data to estimate the density of the postshock gas. Both remnants show asymmetries in the infrared images, and we interpret bright spots as places where the forward shock is running into material that is several times denser than elsewhere. The densities we infer for these objects depend on the grain omposition assumed, and we explore the effects of differing grain porosity on the model fits. We also analyze archival {\it XMM-Newton} RGS spectroscopic data, where both SNRs show strong lines of both Fe and Si, coming from ejecta, as well as strong O lines, which may come from ejecta or shocked ambient medium. We use model fits to IRS spectra to predict X-ray O line strengths for various grain models and values of the shock compression ratio. For 0509-67.5, we find that compact (solid) grain models require nearly all O lines in X-ray spectra to originate in reverse-shocked ejecta. Porous dust grains would lower the strength of ejecta lines relative to those arising in the shocked ambient medium. In 0519-69.0, we find significant evidence for a higher than standard compression ratio of 12, implying efficient cosmic-ray acceleration by the blast wave. A compact grain model is favored over porous grain models. We find that the dust-to-gas mass ratio of the ambient medium is significantly lower than what is expected in the ISM., Accepted for publication in ApJ. 47 pages, 16 figures

We present Wide Field Camera 3 images taken with the Hubble Space Telescope within a single field in the southern grand design star-forming galaxy M83. Based on their size, morphology and photometry in continuum-subtracted H$��$, [\SII], H$��$, [\OIII] and [\OII] filters, we have identified 60 supernova remnant candidates, as well as a handful of young ejecta-dominated candidates. A catalog of these remnants, their sizes and, where possible their H$��$ fluxes are given. Radiative ages and pre-shock densities are derived from those SNR which have good photometry. The ages lie in the range $2.62 < log(��_{\rm rad}/{\rm yr}) < 5.0$, and the pre-shock densities at the blast wave range over $0.56 < n_0/{\rm cm^{-3}} < 1680$. Two populations of SNR have been discovered. These divide into a nuclear and spiral arm group and an inter-arm population. We infer an arm to inter-arm density contrast of 4. The surface flux in diffuse X-rays is correlated with the inferred pre-shock density, indicating that the warm interstellar medium is pressurised by the hot X-ray plasma. We also find that the interstellar medium in the nuclear region of M83 is characterized by a very high porosity and pressure and infer a SNR rate of one per 70-150 yr for the nuclear ($R, Accepted for publication in Astrophysical Journal. 10 figures, 41 pp

Using archival RXTE data, we show that the ultracompact X-ray binary in NGC 1851 exhibits large amplitude X-ray flux varations of more than a factor of 10 on timescales of days to weeks and undergoes sustained periods of months where the time-averaged luminosty varies by factors of two. Variations of this magnitude and timescale have not been reported previously in other ultracompact X-ray binaries. Mass transfer in ultracompact binaries is thought to be driven by gravitational radiation and the predicted transfer rates are so high that the disks of ultracompact binaries with orbits as short as that of this object should not be susceptible to ionization instabilities. Therefore the variability characteristics we observe were unexpected, and need to be understood. We briefly discuss a few alternatives for producing the observed variations in light of the fact that the viscous timescale of the disk is of order a week, comparable to the shorter time scale variation that is observed but much less than the longer term variation. We also discuss the implications for interpretation of observations of extragalactic binaries if the type of variability seen in the source in NGC 1851 is typical., Comment: 7 pages, 5 figures, accepted for publication in MNRAS

We present time-resolved FUV spectra of the dwarf novae SS Cyg and WX Hyi in quiescence from observations using the Hopkins Ultraviolet Telescope on the Astro-1 and Astro-2 Space Shuttle missions and the Goddard High Resolution Spectrograph on the Hubble Space Telescope. Both dwarf novae are characterized by blue continua that extend to the Lyman limit punctuated by broad emission lines including transitions of O VI, N V, Si IV, and C IV. The continuum of WX Hyi can be fit with a white dwarf model with physically reasonable model parameters, but neither system actually shows unambiguous signatures of white dwarf emission. The shape and flux of the spectrum of SS Cyg cannot be self-consistently reconciled with a white dwarf providing all of the FUV continuum flux. Combination white dwarf/disk or white dwarf/optically thin plasma models improve the fit but still do not give physically reasonable model parameters for a quiescent dwarf nova. Assuming that the UV emission lines arise from the disk, the line shapes indicate that surface fluxes fall roughly as R^{-2} in both systems. Fits to the double-peaked line profiles in SS Cyg indicate that the FUV line forming region is concentrated closer to the white dwarf than that of the optical lines and provide no evidence of a hole in the inner disk. Although the flux from SS Cyg was relatively constant during all of our observations, WX Hyi showed significant variability during the GHRS observations. In WX Hyi, the line and continuum fluxes are (with the exception of He II) highly correlated, indicating a link between the formation mechanisms of the line and continuum regions., 20 pages, 11 figures, 4 tables; accepted for publication in ApJ

We have used Hubble Space Telescope to obtain moderate resolution 1150-5700 Ang. spectroscopy of the nucleus of M33 and a blue star ~1 arcsec NNW of the nucleus in an attempt to find the optical counterpart of the nuclear X-ray source and to characterize stellar populations in the nuclear region. The STIS spectra of the nucleus can be modelled in terms of two starbursts, one with a mass of about 9000 solar masses at 40 Myrs and the other with a mass of about 76,000 solar masses at 1 Gyrs. The blue star is a late type O giant, with no obvious spectral anomalies to indicate that it is associated with the luminous X-ray source. The nuclear region is not heavily reddened; 2200 Ang absorption features in the spectra of both the nucleus and the star are weak. The data and the star formation history support the hypothesis that the M33 nuclear source, the brightest persistent source in the Local group, is an ~10 solar mass black hole binary., Comment: 24 pages, including 6 figures. accepted for publication in ApJ, v569 (April 10, 2002)

We present a Hubble Space Telescope/WFPC2 narrow-band H-alpha image of a region on the northeastern limb of the Cygnus Loop supernova remnant. This location provides a detailed example of where the primary blast wave first encounters the surrounding interstellar medium. The filament structure is seen in exquisite detail in this image, which was obtained primarily as an EARLY ACQuisition image for a follow-up spectroscopic program. We compare the HST image to a digitized version of the POSS-I red plate to measure the proper motion of this filament. By combining this value for the proper motion with previous measurements of the shock velocity at this position we find that the distance to the Cygnus Loop is 440 (+130, -100) pc, considerably smaller than the canonical value of 770 pc. We briefly discuss the ramifications of this new distance estimate for our understanding of this prototypical supernova remnant., 18 pages, 3 Figures (2 JPEG and one Postscript)

We present the UBVRI light curves of the Type Ia supernova SN 1998bu which appeared in the nearby galaxy M96 (NGC 3368). M96 is a spiral galaxy in the Leo I group which has a Cepheid-based distance. Our photometry allows us to calculate the absolute magnitude and reddening of this supernova. These data, when combined with measurements of the four other well-observed supernovae with Cepheid based distances, allow us to calculate the Hubble constant with respect to the Hubble flow defined by the distant Calan/Tololo Type Ia sample. We find a Hubble constant of 64.0 +/- 2.2(internal) +/- 3.5(external) km/s/Mpc, consistent with most previous estimates based on Type Ia supernovae. We note that the two well-observed Type Ia supernovae in Fornax, if placed at the Cepheid distance to the possible Fornax spiral NGC 1365, are apparently too faint with respect to the Calan/Tololo sample calibrated with the five Type Ia supernovae with Cepheid distances to the host galaxies., Comment: AAS LaTeX, 20 pages, 4 figures, 6 tables, accepted for publication in the Astronomical Journal. Figure 1 (finding chart) not included

In October 2018, Swift announced the discovery of a new Galactic X-ray transient, Swift J1858. Just before Sun-angle constraints rendered the system unobservable, follow-up observations revealed extreme flaring activity, of a kind that has so far only been seen in the famous black hole X-ray binary (BHXRB) V404 Cyg during its 2015 eruption and in V4641 Sgr. The peculiar behaviour of these sources is thought to be a consequence of super-Eddington accretion regime. After several months of unusual strong and rapid flaring in its high-luminosity state, Swift J1858 is currently exhibiting impressive optical P-Cygni profiles, suggesting the pres- ence of a dense and cool wind from the outer accretion disk. The dominant spectroscopic signatures of such winds are actually expected to lie in the far-ultraviolet region, but they are usually inaccessible in black-hole X-ray binaries, due to interstellar reddening. Given its low extinction, Swift J1858 provides us with a rare chance to study the accretion disk wind in the crucial ultraviolet band - an opportunity that was missed in the other two systems. Building on an ongoing multi-wavelength campaign (X-rays: NICER; optical: GTC; radio: VLA & AMI), we therefore request far- and near-UV time-resolved spectroscopic observations of this system with HST/STIS+COS in order to (a) study its extreme accretion disk wind; (b) test proposed wind driving mechanisms; (c) characterize its UV variability properties and determine the origin of these variations; (d) construct the broad-band SED of the outer accretion disk that dominates the UV flux; and (e) determine the extinction towards the system in order to constrain the mass accretion rate.

Non-radial pulsations have been identified in a number of accreting white dwarfs in cataclysmic variables. These stars offer insight into the excitation of pulsation modes in mixed H/He/Z atmospheres, and the response of these modes to changes in the white dwarf temperature. Among all pulsating cataclysmic variable white dwarfs, GW Lib stands out by having a well-established observational record of three independent pulsation modes that were wiped out during its 2007 outburst. We have obtained new HST ultraviolet observations in May 2013 that show an unexpected behaviour: besides some activity near the ̃280 s period that has been observed in the past, the white dwarf underwent a large-amplitude brightening. We demonstrate that the brightening is related to an increase of the photospheric temperature, argue against an accretion episode as explanation, and discuss this event in the context of non-radial pulsations on a rapidly rotating star.

A survey that can cover the sky in optical bands over wide fields to faint magnitudes with a fast cadence will enable many of the exciting science opportunities of the next decade. The Large Synoptic Survey Telescope (LSST) will have an effective aperture of 6.7 meters and an imaging camera with field of view of 9.6 deg^2, and will be devoted to a ten-year imaging survey over 20,000 deg^2 south of +15 deg. Each pointing will be imaged 2000 times with fifteen second exposures in six broad bands from 0.35 to 1.1 microns, to a total point-source depth of r~27.5. The LSST Science Book describes the basic parameters of the LSST hardware, software, and observing plans. The book discusses educational and outreach opportunities, then goes on to describe a broad range of science that LSST will revolutionize: mapping the inner and outer Solar System, stellar populations in the Milky Way and nearby galaxies, the structure of the Milky Way disk and halo and other objects in the Local Volume, transient and variable objects both at low and high redshift, and the properties of normal and active galaxies at low and high redshift. It then turns to far-field cosmological topics, exploring properties of supernovae to z~1, strong and weak lensing, the large-scale distribution of galaxies and baryon oscillations, and how these different probes may be combined to constrain cosmological models and the physics of dark energy., 596 pages. Also available at full resolution at http://www.lsst.org/lsst/scibook