Search

We present three epochs of early-time ultraviolet (UV) and optical HST/STIS spectroscopy of the young, nearby Type IIP supernova (SN) 2021yja. We complement the HST data with two earlier epochs of Swift UVOT spectroscopy. The HST and Swift UVOT spectra are consistent with those of other well-studied Type IIP SNe. The UV spectra exhibit rapid cooling at early times, while less dramatic changes are seen in the optical. We also present Lick/KAIT optical photometry up to the late-time tail phase, showing a very long plateau and shallow decline compared with other SNe IIP. Our modeling of the UV spectrum with the TARDIS radiative transfer code produces a good fit for a high-velocity explosion, a low total extinction E (B âˆ' V) = 0.07 mag, and a subsolar metallicity. We do not find a significant contribution to the UV flux from an additional heating source, such as interaction with the circumstellar medium, consistent with the observed flat plateau. Furthermore, the velocity width of the Mg ii λ 2798 line is comparable to that of the hydrogen Balmer lines, suggesting that the UV emission is confined to a region close to the photosphere. [ABSTRACT FROM AUTHOR]

We present a test of the statistical method introduced by Bernard F. Shutz in 1986 using only gravitational waves to infer the Hubble constant (H0) from GW190814, the first high-probability neutron-star–black hole (NS–BH) merger candidate detected by the Laser Interferometer gravitational-wave Observatory (LIGO) and the Virgo interferometer. We apply a baseline test of this method to the binary neutron star (BNS) merger GW170817 and find km s−1 Mpc−1 (maximum a posteriori and 68.3% highest density posterior interval) for a galaxy B-band luminosity threshold of with a correction for catalog incompleteness. Repeating the calculation for GW190814, we obtain km s−1 Mpc−1 and km s−1 Mpc−1 for and , respectively. Combining the posteriors for both events yields km s−1 Mpc−1, demonstrating the improvement on constraints when using multiple gravitational-wave events. We also confirm the results of other works that adopt this method, showing that increasing the LB threshold enhances the posterior structure and slightly shifts the distribution's peak to higher H0 values.We repeat the joint inference using the low-spin PhenomPNRT and the newly available combined (SEOBNRv4PHM + IMRPhenomPv3HM) posterior samples for GW170817 and GW190814, respectively, achieving a tighter constraint of km s−1 Mpc−1. [ABSTRACT FROM AUTHOR]

We present the complete sample of stripped-envelope supernova (SN) spectra observed by the Lick Observatory Supernova Search (LOSS) collaboration over the last three decades: 888 spectra of 302 SNe, 652 published here for the first time, with 384 spectra (of 92 SNe) having photometrically determined phases. After correcting for redshift and Milky Way dust reddening and reevaluating the spectroscopic classifications for each SN, we construct mean spectra of the three major spectral subtypes (Types IIb, Ib, and Ic) binned by phase. We compare measures of line strengths and widths made from this sample to the results of previous efforts, confirming that O i λ7774 absorption is stronger and found at higher velocity in Type Ic SNe than in Types Ib or IIb SNe in the first ∼30 days after peak brightness, though the widths of nebular emission lines are consistent across subtypes. We also highlight newly available observations for a few rare subpopulations of interest. [ABSTRACT FROM AUTHOR]

Late-time spectra of Type Ia supernovae (SNe Ia) are important in clarifying the physics of their explosions, as they provide key clues to the inner structure of the exploding white dwarfs. We examined late-time optical spectra of 36 SNe Ia, including five from our own project (SNe 2019np, 2019ein, 2021hpr, 2021wuf, and 2022hrs), with phase coverage of ∼200 to ∼400 d after maximum light. At this late phase, the outer ejecta have become transparent and the features of inner iron-group elements emerge in the spectra. Based on multicomponent Gaussian fits and reasonable choices for the pseudo-continuum around Ni and Fe emission features, we get reliable estimates of the Ni to Fe ratio, which is sensitive to the explosion models of SNe Ia. Our results show that the majority (about 67 per cent) of our SNe Ia are more consistent with the sub-Chandrasekhar-mass (i.e. double-detonation) model, although they could be affected by evolutionary or ionization effects. Moreover, we find that the Si  ii λ6355 velocity measured around the time of maximum light tends to increase with the Ni to Fe ratio for the subsample with either redshifted or blueshifted nebular velocities, suggesting that progenitor metallicity might play an important role in accounting for the observed velocity diversity of SNe Ia. [ABSTRACT FROM AUTHOR]

The luminosities of type Ia supernovae (SNe), the thermonuclear explosions of white-dwarf stars, vary systematically with their intrinsic color and the rate at which they fade. From images taken with the Galaxy Evolution Explorer (GALEX), we identified SNe Ia that erupted in environments that have high ultraviolet surface brightness and star-formation surface density. When we apply a steep model extinction law, we calibrate these SNe using their broadband optical light curves to within ~0.065 to 0.075 magnitude, corresponding to <4% in distance. The tight scatter, probably arising from a small dispersion among progenitor ages, suggests that variation in only one progenitor property primarily accounts for the relationship between their light-curve widths, colors, and luminosities. [ABSTRACT FROM AUTHOR]

ABSTRACT While O is often seen in spectra of Type Ia supernovae (SNe Ia) as both unburned fuel and a product of C burning, C is only occasionally seen at the earliest times and represents the most direct way of investigating primordial white dwarf material and its relation to SN Ia explosion scenarios and mechanisms. In this paper, we search for C absorption features in 188 optical spectra of 144 low-redshift ( z < 0.1) SNe Ia with ages ≲3.6 d after maximum brightness. These data were obtained as part of the Berkeley Supernova Ia Program (BSNIP) and represent the largest set of SNe Ia in which C has ever been searched. We find that ∼11 per cent of the SNe studied show definite C absorption features, while ∼25 per cent show some evidence for C ii in their spectra. Also, if one obtains a spectrum at ≲ -5 d, then there is a better than 30 per cent chance of detecting a distinct absorption feature from C ii. SNe Ia that show C are found to resemble those without C in many respects, but objects with C tend to have bluer optical colours than those without C. The typical expansion velocity of the C ii λ6580 feature is measured to be 12 000-13 000 km s−1, and the ratio of the C ii λ6580 to Si ii λ6355 velocities is remarkably constant with time and among different objects with a median value of ∼1.05. While the pseudo-equivalent widths (pEWs) of the C ii λλ6580 and 7234 features are found mostly to decrease with time, we see evidence of a significant increase in pEW between ∼12 and 11 d before maximum brightness, which is actually predicted by some theoretical models. The range of pEWs measured from the BSNIP data implies a range of C masses in SN Ia ejecta of about (2-30) × 10−3 M⊙. [ABSTRACT FROM AUTHOR]

I present an overview of optical observations of Type II, IIb, Ib, and Ic supernovae (SNe), all of which are thought to arise from core collapse in massive progenitors that have previously experienced different amounts of mass loss. SNe Ic appear to have been stripped the most, followed by SNe Ib and SNe IIb. In the case of SNe Iln, part of the hydrogen envelope remains, yet the ejecta interact with unusually dense circumstellar gas, probably a wind from the progenitor star. Some SNe IIn, however, might not be genuine supernovae, but rather "super-outbursts" of luminous variable stars. Spectropolarimetry of core-collapse SNe reveals that asphericity tends to increase toward the core. It has recently been shown that several long-duration gamma-ray bursts were undeniably associated with peculiar SNe Ic having exceptionally high ejecta velocities. [Copyright &y& Elsevier]

We present new Hubble Space Telescope (HST) imaging photometry for the site of the Type IIn supernova (SN) 2009ip taken almost a decade after explosion. The optical source has continued to fade steadily since the SN-like event in 2012. In the F606W filter, which was also used to detect its luminous blue variable (LBV) progenitor 13 yr before the SN, the source at the position of SN 2009ip is now 1.2 mag fainter than that quiescent progenitor. It is 6–7 mag fainter than the pre-SN outbursts in 2009–2011. This definitively rules out a prediction that the source would return to its previous state after surviving the 2012 event. Instead, the late-time fading matches expectations for a terminal explosion. The source fades at a similar rate in all visual-wavelength filters without significant colour changes, therefore also ruling out the hypothesis of a luminous dust-obscured survivor or transition to a hotter post-LBV survivor. The late-time continuum with steady colour and strong Hα emission detected in a narrow F657N filter are, however, entirely expected for ongoing shock interaction with circumstellar material in a decade-old core-collapse SN. Interestingly, the ultraviolet flux has stayed nearly constant since 2015, supporting previous conjectures that the F275W light traces main-sequence OB stars in an underlying young star cluster. We expect that the visual-wavelength continuum will eventually level off, tracing this cluster light. Without any additional outbursts, it seems prudent to consider the 2012 event as a terminal SN explosion, and we discuss plausible scenarios. [ABSTRACT FROM AUTHOR]

Galaxy-cluster gravitational lenses enable the study of faint galaxies even at large lookback times, and, recently, time-delay constraints on the Hubble constant. There have been few tests, however, of lens model predictions adjacent to the critical curve (≲8″) where the magnification is greatest. In a companion paper, we use the GLAFIC lens model to constrain the Balmer L – σ relation for H ii regions in a galaxy at redshift z = 1.49 strongly lensed by the MACS J1149 galaxy cluster. Here we perform a detailed comparison between the predictions of 10 cluster lens models that employ multiple modeling assumptions with our measurements of 11 magnified, giant H ii regions. We find that that the models predict magnifications an average factor of 6.2 smaller, a ∼2 σ tension, than that inferred from the H ii regions under the assumption that they follow the low-redshift L – σ relation. To evaluate the possibility that the lens model magnifications are strongly biased, we next consider the flux ratios among knots in three images of Sp1149, and find that these are consistent with model predictions. Moreover, while the mass-sheet degeneracy could in principle account for a factor of ∼6 discrepancy in magnification, the value of H 0 inferred from SN Refsdal's time delay would become implausibly small. We conclude that the lens models are not likely to be highly biased, and that instead the H ii regions in Sp1149 are substantially more luminous than the low-redshift Balmer L – σ relation predicts. [ABSTRACT FROM AUTHOR]

The origins of quasi-periodic eruptions (QPEs) are poorly understood, although most theoretical explanations invoke an accretion disc around a supermassive black hole. The gas and stellar environments in the galactic nuclei of these sources are also poorly constrained. In this paper, we present an analysis of archival Hubble Space Telescope (HST) images to study the narrow-line [O  iii ] emission in the QPE source GSN 069. We find strong evidence for a compact nuclear [O  iii ] emission region of size ≲ 35 pc, overlaid on top of extended [O  iii ] emission up to 1 kpc away from the nucleus. The age of the accretion system is estimated to be between 10 and 100 yr. The [O  iii ] luminosity of the compact region was measured to be |$(2.1 \pm 0.3) \times 10^{40}\, \rm erg\, s^{-1}$|⁠. Based on cloudy simulations, we constrain that the [O  iii ] emitting gas has a hydrogen number density in the range |$2 \times 10^{3} \lt n_{\rm H} \lesssim 10^{8}\, \rm cm^{-3}$| and volume filling factor f V < 4 × 10−3. We suggest that the dense gas in the nuclear region of GSN 069 originates from molecular clouds (with total mass |$\gtrsim 3 \times 10^{3}\, {\rm M}_{\odot }$|⁠), which are freshly ionized by the soft X-ray photons from the accretion disc. We predict possible evolution of the compact narrow-line region on emission-line diagnostic diagrams, and hence future HST or integral-field unit observations can be used to further pin down the age of this puzzling system. [ABSTRACT FROM AUTHOR]

A growing number of supernovae (SNe) are now known to exhibit evidence for significant interaction with a dense, pre-existing, circumstellar medium (CSM). SNe Ibn comprise one such class that can be characterized by both rapidly evolving light curves and persistent narrow He  i lines. The origin of such a dense CSM in these systems remains a pressing question, specifically concerning the progenitor system and mass-loss mechanism. In this paper, we present multiwavelength data of the Type Ibn SN 2020nxt, including HST /STIS ultraviolet spectra. We fit the data with recently updated CMFGEN models designed to handle configurations for SNe Ibn. The UV coverage yields strong constraints on the energetics and, when combined with the CMFGEN models, offer new insight on potential progenitor systems. We find the most successful model is a ≲4 M⊙ helium star that lost its |$\sim 1\, {\rm M}_\odot$| He-rich envelope in the years preceding core collapse. We also consider viable alternatives, such as a He white dwarf merger. Ultimately, we conclude at least some SNe Ibn do not arise from single, massive (>30 M⊙) Wolf–Rayet-like stars. [ABSTRACT FROM AUTHOR]

We present photometry and spectroscopy of the slowly evolving superluminous Type IIn supernova (SN) 2015da. SN 2015da is extraordinary for its very high peak luminosity, and also for sustaining a high luminosity for several years. Even at 8 yr after explosion, SN 2015da remains as luminous as the peak of a normal SN II-P. The total radiated energy integrated over this time period (with no bolometric correction) is at least |$1.6 \times 10^{51}$| erg (or 1.6 FOE). Including a mild bolometric correction, adding kinetic energy of the expanding cold dense shell of swept-up circumstellar material (CSM), and accounting for asymmetry, the total explosion kinetic energy was likely 5–10 FOE. Powering the light curve with CSM interaction requires an energetic explosion and 20 M |$_{\odot }$| of H-rich CSM, which in turn implies a massive progenitor system |$\gt $| 30 M |$_{\odot }$|⁠. Narrow P Cyg features show steady CSM expansion at 90 km s |$^{-1}$|⁠ , requiring a high average mass-loss rate of |$\sim$| 0.1 M |$_{\odot }$|  yr |$^{-1}$| sustained for two centuries before explosion (although ramping up toward explosion time). No current theoretical model for single-star pre-SN mass-loss can account for this. The slow CSM, combined with broad wings of H  |$\alpha$| indicating H-rich material in the unshocked ejecta, disfavours a pulsational pair instability model for the pre-SN mass-loss. Instead, violent pre-SN binary interaction is a likely culprit. Finally, SN 2015da exhibits the characteristic asymmetric blueshift in its emission lines from shortly after peak until the present epoch, adding another well-studied superluminous SNe IIn with unambiguous evidence of post-shock dust formation. [ABSTRACT FROM AUTHOR]

The temporal evolution of the optical spectra of various types of supernovae (SNe) is illustrated, in part to aid observers classifying supernova candidates. Type II SNe are defined by the presence of hydrogen, and they exhibit a very wide variety of photometric and spectroscopic properties. Among hydrogen-deficient SNe (Type I), three subclasses are now known: those whose early-time spectra show strong Si II (Ia), prominent He I (Ib), or neither Si II nor He I (Ic). The late-time spectra of SNe Ia consist of a multitude of blended emission lines of iron-group elements; in sharp contrast, those of SNe Ib and SNe Ic (which are similar to each other) are dominated by several relatively unblended lines of intermediate-mass elements. Although SNe Ia, which result from the thermonuclear runaway of white dwarfs, constitute a rather homogeneous subclass, important variations in their photometric and spectroscopic properties are undeniably present. SNe Ib/Ic probably result from core collapse in massive stars largely stripped of their hydrogen (Ib) and helium (Ic) envelopes, and hence they are physically related to SNe II. Indeed, the progenitors of some SNe II seem to have only a low-mass skin of hydrogen; their spectra gradually evolve to resemble those of SNe Ib. In addition to the two well-known photometric subclasses (linear and plateau) of SNe II, which may exhibit minor spectroscopic differences, there is a new subclass (SNe IIn) distinguished by relatively narrow emission lines with little or no P Cygni absorption component and slowly declining light curves. These objects probably have unusually dense circumstellar gas with which the ejecta interact. [ABSTRACT FROM AUTHOR]

An important and perhaps critical clue to the mechanism driving the explosion of massive stars as supernovae is provided by the accumulating evidence for asymmetry in the explosion. Indirect evidence comes from high pulsar velocities, associations of supernovae with long-soft γ-ray bursts, and asymmetries in late-time emission-line profiles. Spectropolarimetry provides a direct probe of young supernova geometry, with higher polarization generally indicating a greater departure from spherical symmetry. Large polarizations have been measured for ‘stripped-envelope’ (that is, type Ic; ref. 7) supernovae, which confirms their non-spherical morphology; but the explosions of massive stars with intact hydrogen envelopes (type II-P supernovae) have shown only weak polarizations at the early times observed. Here we report multi-epoch spectropolarimetry of a classic type II-P supernova that reveals the abrupt appearance of significant polarization when the inner core is first exposed in the thinning ejecta (∼90 days after explosion). We infer a departure from spherical symmetry of at least 30 per cent for the inner ejecta. Combined with earlier results, this suggests that a strongly non-spherical explosion may be a generic feature of core-collapse supernovae of all types, where the asphericity in type II-P supernovae is cloaked at early times by the massive, opaque, hydrogen envelope. [ABSTRACT FROM AUTHOR]

We present mid-infrared (mid-IR) imaging of the Type IIL supernova (SN) 1980K with the JWST more than 40 yr post-explosion. SN 1980K, located in the nearby (D ≈ 7 Mpc) 'SN factory' galaxy NGC 6946, was serendipitously captured in JWST /MIRI images taken of the field of SN 2004et in the same galaxy. SN 1980K serves as a promising candidate for studying the transitional phase between young SNe and older SN remnants and also provides a great opportunity to investigate its the close environment. SN 1980K can be identified as a clear and bright point source in all eight MIRI filters from F 560 W up to F 2550 W. We fit analytical dust models to the mid-IR spectral energy distribution that reveal a large amount (M d ≈ 0.002 M⊙) of Si-dominated dust at |$T_\rm {dust}\approx 150$|  K (accompanied by a hotter dust/gas component), and also computed numerical SED dust models. Radiative transfer modelling of a late-time optical spectrum obtained recently with Keck discloses that an even larger (∼0.24–0.58 M⊙) amount of dust is needed in order for selective extinction to explain the asymmetric line profile shapes observed in SN 1980K. As a conclusion, with JWST , we may see i) pre-existing circumstellar dust heated collisionally (or, partly radiatively), analogous to the equatorial ring of SN 1987A, or ii) the mid-IR component of the presumed newly-formed dust, accompanied by much more colder dust present in the ejecta (as suggested by the late-time the optical spectra). [ABSTRACT FROM AUTHOR]

We present new spectroscopic and photometric follow-up observations of the known sample of extreme coronal line-emitting galaxies (ECLEs) identified in the Sloan Digital Sky Survey (SDSS). With these new data, observations of the ECLE sample now span a period of two decades following their initial SDSS detections. We confirm the non-recurrence of the iron coronal line signatures in five of the seven objects, further supporting their identification as the transient light echoes of tidal disruption events (TDEs). Photometric observations of these objects in optical bands show little overall evolution. In contrast, mid-infrared (MIR) observations show ongoing long-term declines consistent with power-law decay. The remaining two objects had been classified as active galactic nuclei (AGNs) with unusually strong coronal lines rather than being TDE related, given the persistence of the coronal lines in earlier follow-up spectra. We confirm this classification, with our spectra continuing to show the presence of strong, unchanged coronal line features and AGN-like MIR colours and behaviour. We have constructed spectral templates of both subtypes of ECLE to aid in distinguishing the likely origin of newly discovered ECLEs. We highlight the need for higher cadence, and more rapid, follow-up observations of such objects to better constrain their properties and evolution. We also discuss the relationships between ECLEs, TDEs, and other identified transients having significant MIR variability. [ABSTRACT FROM AUTHOR]

The Tsinghua University-Ma Huateng Telescope for Survey (TMTS) has been constantly monitoring the northern sky since 2020 in search of rapidly variable stars. To find variable white dwarfs (WDs), the TMTS catalogue is cross-matched with the WD catalogue of Gaia EDR3, resulting in over 3000 light curves of WD candidates. The WD TMTS J17184064+2524314 (hereafter J1718) is the second ZZ Ceti star discovered among these common sources. Based on the light curves from TMTS, follow-up photometric observations, and Transiting Exoplanet Survey Satellite , 10 periods and three combination periods are detected. A rotation period of 25.12 ± 0.18 h is derived, according to the identified rotational splitting. Our spectroscopic observation indicates that this WD belongs to DA type with T eff = 11 670 ± 604 K, log  g = 8.16 ± 0.36, M = 0.70 ± 0.23 M⊙, and age = 0.51 ± 0.34 Gyr. Based on core-parametrized asteroseismological model grids (≥14 million), we derive the best-fitting solution of T eff = 11 640 ± 20 K, log  g = 8.267 ± 0.008, and M = 0.750 ± 0.005 M⊙ for J1718, consistent with the spectral fitting results. For this WD, the corresponding carbon and oxygen abundances in the core are 0.43 and 0.57, respectively. The distance derived from the intrinsic luminosity given by asteroseismology is 64 ± 15 pc, in accord with the distance of 70.1 ± 0.2 pc from Gaia DR3 within the uncertainties. [ABSTRACT FROM AUTHOR]

We present the discovery and analysis of SN 2022oqm, a Type Ic supernova (SN) detected <1 day after the explosion. The SN rises to a blue and short-lived (2 days) initial peak. Early-time spectral observations of SN 2022oqm show a hot (40,000 K) continuum with high ionization C and O absorption features at velocities of 4000 km s−1, while its photospheric radius expands at 20,000 km s−1, indicating a pre-existing distribution of expanding C/O material. After ∼2.5 days, both the spectrum and light curves evolve into those of a typical SN Ic, with line velocities of ∼10,000 km s−1, in agreement with the evolution of the photospheric radius. The optical light curves reach a second peak at t ≈ 15 days. By t = 60 days, the spectrum of SN 2022oqm becomes nearly nebular, displaying strong Ca ii and [Ca ii ] emission with no detectable [O i ], marking this event as Ca-rich. The early behavior can be explained by 10−3 M ⊙ of optically thin circumstellar material (CSM) surrounding either (1) a massive compact progenitor such as a Wolf–Rayet star, (2) a massive stripped progenitor with an extended envelope, or (3) a binary system with a white dwarf. We propose that the early-time light curve is powered by both the interaction of the ejecta with the optically thin CSM and shock cooling (in the massive star scenario). The observations can be explained by CSM that is optically thick to X-ray photons, is optically thick in the lines as seen in the spectra, and is optically thin to visible-light continuum photons that come either from downscattered X-rays or from the shock-heated ejecta. Calculations show that this scenario is self-consistent. [ABSTRACT FROM AUTHOR]

We present extensive photometric and spectroscopic observations of the peculiar Type Ia supernova (SN Ia) 2022vqz. It shares many similarities with the SN 2002es-like SNe Ia, such as low luminosity (⁠|$M_{B,\rm max}=-18.11\pm 0.16$| mag) and moderate post-peak decline rate (Δ m 15, B  = 1.33 ± 0.11 mag). The nickel mass synthesized in the explosion is estimated as 0.20 ± 0.04 M⊙ from the bolometric light curve, which is obviously lower than that of normal SNe Ia. SN 2022vqz is also characterized by slowly expanding ejecta, with Si  ii velocities persisting around 7000 km s−1 since 16 d before peak brightness, unique among all known SNe Ia. While all of these properties imply a lower energy thermonuclear explosion that should leave a considerable amount of unburnt materials, the absent signature of unburnt carbon in spectra of SN 2022vqz is puzzling. A prominent early peak is clearly detected in the Asteroid Terrestrial-impact Last Alert System c - and o -band light curves and in the Zwicky Transient Facility gr -band data within days after the explosion. Possible mechanisms for the early peak are discussed, including the sub-Chandrasekhar-mass double-detonation model and interaction of SN ejecta with circumstellar material. We find that both models face some difficulties in replicating all aspects of the observed data. As an alternative, we propose a hybrid C–O–Ne white dwarf as the progenitor of SN 2022vqz; it can simultaneously reconcile the tension between low ejecta velocity and the absence of carbon. We further discuss the diversity of SN 2002es-like objects and their origin in the context of different scenarios. [ABSTRACT FROM AUTHOR]

We present multi-epoch spectropolarimetry of Type IIn supernova SN2017hcc, 16–391 d after explosion. Continuum polarization up to 6 per cent is observed during the first epoch, making SN 2017hcc the most intrinsically polarized SN ever reported at visible wavelengths. During the first 29 d, when the polarization is strongest, the continuum polarization exhibits wavelength dependence that rises toward the blue, then becomes wavelength independent by day 45. The polarization drops rapidly during the first month, even as the flux is still climbing to peak brightness. None the less, unusually high polarization is maintained until day 68, at which point the polarization declines to levels comparable to those of previous well-studied SNe IIn. Only minor changes in position angle (PA) are measured throughout the evolution. The blue slope of the polarized continuum and polarized line emission during the first month suggests that an aspherical distribution of dust grains in pre-shock circumstellar material (CSM) is echoing the SN IIn spectrum and strongly influencing the polarization, while the subsequent decline during the wavelength-independent phase appears consistent with electron scattering near the SN/CSM interface. The persistence of the PA between these two phases suggests that the pre-existing CSM responsible for the dust scattering at early times is part of the same geometric structure as the electron-scattering region that dominates the polarization at later times. SN 2017hcc appears to be yet another, but more extreme, case of aspherical yet well-ordered CSM in Type IIn SNe, possibly resulting from pre-SN mass-loss shaped by a binary progenitor system. [ABSTRACT FROM AUTHOR]

We present six epochs of optical spectropolarimetry of the Type IIP supernova (SN) 2021yja ranging from ∼25 to 95 d after the explosion. An unusually high continuum linear polarization of |$p \approx 0.9~{{\ \rm per\ cent}}$| is measured during the early photospheric phase, followed by a steady decrease well before the onset of the nebular phase. This behaviour has not been observed before in Type IIP supernovae (SNe IIP). The observed continuum polarization angle does not change significantly during the photospheric phase. We find a pronounced axis of symmetry in the global ejecta that is shared in common with the Hα and Ca  ii near-infrared triplet lines. These observations are consistent with an ellipsoidal geometry. The temporal evolution of the continuum polarization is also compatible with the SN ejecta interacting with aspherical circumstellar matter (CSM), although no spectroscopic features that may be associated with strong interaction can be identified. Alternatively, we consider the source of the high polarization to be an extended hydrogen envelope that is indistinguishable from low-density CSM. [ABSTRACT FROM AUTHOR]

We present the first photometric analysis of the supernova (SN) impostor AT 2016blu in NGC 4559. This transient was discovered by the Lick Observatory Supernova Search in 2012 and has continued its outbursts since then. Optical and infrared photometry of AT 2016blu reveals at least 19 outbursts in 2012–2022. Similar photometry from 1999 to 2009 shows no outbursts, indicating that the star was relatively stable in the decade before discovery. Archival Hubble Space Telescope observations suggest that the progenitor had a minimum initial mass of |$M \gtrsim 33$| M⊙ and a luminosity of |$L \gtrsim 10^{5.7}$| L⊙. AT 2016blu's outbursts show irregular variability with multiple closely spaced peaks having typical amplitudes of 1–2 mag and durations of 1–4 weeks. While individual outbursts have irregular light curves, concentrations of these peaks recur with a period of ∼113 ± 2 d. Based on this period, we predict upcoming outbursts in 2023 and 2024. AT 2016blu shares similarities with SN 2000ch in NGC 3432, where outbursts may arise from periastron encounters in an eccentric binary containing a luminous blue variable (LBV). We propose that AT 2016blu's outbursts are also driven by interactions that intensify around periastron in an eccentric system. Intrinsic variability of the LBV-like primary star may cause different intensity and duration of binary interaction at each periastron passage. AT 2016blu also resembles the periastron encounters of η Carinae prior to its Great Eruption and the erratic pre-SN eruptions of SN 2009ip. This similarity and the onset of eruptions in the past decade hint that AT 2016blu may also be headed for a catastrophe, making it a target of great interest. [ABSTRACT FROM AUTHOR]

We analyze the photometry and spectra of the "redback" millisecond pulsar binary J2339−0533. These observations include new measurements from Keck and the Gamma-Ray Burst Optical/Near-infrared Detector (GROND), as well as archival measurements from the Optical and Infrared Synergetic Telescopes for Education and Research (OISTER), WIYN, Southern Astrophysical Research (SOAR), and Hobby–Eberly Telescope (HET) telescopes. The parameters derived from GROND, our primary photometric data, describe well the rest of the data sets, raising our confidence in our fitted binary properties. Our fit requires hot spots (likely magnetic poles) on the surface of the companion star, and we see evidence that these spots move over the 8 yr span of our photometry. The derived binary inclination i = 69.°3 ± 2.°3, together with the center-of-mass velocity (from the radial-velocity fits) KC = 347.0 ± 3.7 , give a fairly typical neutron star mass of 1.47 ± 0.09 M⊙. [ABSTRACT FROM AUTHOR]

We report on analysis using the JWST to identify a candidate progenitor star of the Type II-plateau (II-P) supernova SN 2022acko in the nearby, barred spiral galaxy NGC 1300. To our knowledge, our discovery represents the first time JWST has been used to localize a progenitor system in pre-explosion archival Hubble Space Telescope (HST) images. We astrometrically registered a JWST NIRCam image from 2023 January, in which the SN was serendipitously captured, to pre-SN HST F 160 W and F 814 W images from 2017 and 2004, respectively. An object corresponding precisely to the SN position has been isolated with reasonable confidence. That object has a spectral energy distribution (SED) and overall luminosity consistent with a single-star model having an initial mass possibly somewhat less than the canonical 8 M⊙ theoretical threshold for core collapse (although masses as high as 9 M⊙ for the star are also possible); however, the star's SED and luminosity are inconsistent with that of a super-asymptotic giant branch star that might be a forerunner of an electron-capture SN. The properties of the progenitor alone imply that SN 2022acko is a relatively normal SN II-P, albeit most likely a low-luminosity one. The progenitor candidate should be confirmed with follow-up HST imaging at late times, when the SN has sufficiently faded. This potential use of JWST opens a new era of identifying SN progenitor candidates at high spatial resolution. [ABSTRACT FROM AUTHOR]

NASA will launch the Nancy Grace Roman Space Telescope (hereafter Roman) in the second half of this decade, which will allow for a generation-defining measurement of dark energy through multiple probes, including Type Ia supernovae (SNe Ia). To improve decisions on survey strategy, we have created the first simulations of realistic Roman images that include artificial SNe Ia injected as point sources in the images. Our analysis combines work done on Roman simulations for weak gravitational lensing studies as well as catalogue-level simulations of SN Ia samples. We have created a time series of images over 2 yr containing ∼1050 SNe Ia, covering a 1 deg2 subarea of a planned 5 deg2 deep survey. We have released these images publicly for community use along with input catalogues of all injected sources. We create secondary products from these images by generating coadded images and demonstrating recovery of transient sources using image subtraction. We perform first-use analyses on these images in order to measure galaxy detection efficiency, point source detection efficiency, and host-galaxy association biases. The simulated images can be found here at https://roman.ipac.caltech.edu/sims/SN_Survey_Image_sim.html. [ABSTRACT FROM AUTHOR]

Over the past few years, wide-field time-domain surveys such as Zwicky Transient Facility and Optical Gravitational Lensing Experiment have led to discoveries of various types of interesting short-period stellar variables, such as ultracompact eclipsing binary white dwarfs (WDs), rapidly rotating magnetized WDs, transitional cataclysmic variables between hydrogen-rich and helium accretion, and blue large-amplitude pulsators (BLAPs), which greatly enrich our understandings of stellar physics under some extreme conditions. In this paper, we report the first-2-yr discoveries of short-period variables (i.e. P < 2 h) by the Tsinghua University–Ma Huateng Telescopes for Survey (TMTS). TMTS is a multitube telescope system with a field of view up to 18 deg2, which started to monitor the Large Sky Area Multi-object Fiber Spectroscopic Telescope (LAMOST) sky areas since 2020 and generated uninterrupted minute-cadence light curves for about 10 million sources within 2 yr. Adopting the Lomb–Scargle periodogram with period-dependent thresholds for the maximum powers, we identify over 1100 sources that exhibit a variation period shorter than 2 h. Compiling the light curves with the Gaia magnitudes and colours, LAMOST spectral parameters, International Variable Star Index classifications, and archived observations from other prevailing time-domain survey missions, we identified 1076 as δ Scuti stars, which allows us to study their populations and physical properties in the short-period regime. The other 31 sources include BLAPs, subdwarf B variables, pulsating WDs, ultracompact/short-period eclipsing/ellipsoidal binaries, cataclysmic variables below the period gap, etc. which are highly interesting and worthy of follow-up investigations. [ABSTRACT FROM AUTHOR]

Tsinghua University-Ma Huateng Telescope for Survey (TMTS) aims to discover rapidly evolving transients by monitoring the northern sky. The TMTS catalogue is cross-matched with the white dwarf (WD) catalogue of Gaia EDR3, and light curves of more than a thousand WD candidates are obtained so far. Among them, the WD TMTS J23450729+5813146 is one interesting common source. Based on the light curves from the TMTS and follow-up photometric observations, periods of 967.113, 973.734, 881.525, 843.458, 806.916, and 678.273 s are identified. In addition, the TESS observations suggest a 3.39-h period but this can be attributed to the rotation of a comoving M dwarf located within 3 arcsec. The spectroscopic observation indicates that this WD is DA type with T eff = 11 778 ± 617 K, log  g  = 8.38 ± 0.31, mass = 0.84 ± 0.20 M⊙, and age = 0.704 ± 0.377 Gyr. Asteroseismological analysis reveals a global best-fitting solution of T eff = 12 110 ± 10 K and mass = 0.760 ± 0.005 M⊙, consistent with the spectral fitting results, and oxygen and carbon abundances in the core centre are 0.73 and 0.27, respectively. The distance derived from the intrinsic luminosity given by asteroseismology is 93 parsec, which is in agreement with the distance of 98 parsec from Gaia DR3. Additionally, kinematic study shows that this WD is likely a thick-disc star. The mass of its zero-age main sequence is estimated to be 3.08 M⊙ and has a main sequence plus cooling age of roughly 900 Myr. [ABSTRACT FROM AUTHOR]

We have conducted photometric and spectroscopic observations of the peculiar Type Ia supernova (SN Ia) 2016ije that was discovered through the Tsinghua-NAOC Transient Survey. This peculiar object exploded in the outskirts of a metal-poor, low-surface brightness galaxy (i.e., M g = −14.5 mag). Our photometric analysis reveals that SN 2016ije is subluminous ( M B , max = −17.65 ± 0.06 mag) but exhibits relatively broad light curves (Δ m 15(B) = 1.35 ± 0.14 mag), similar to the behavior of SN 2002es. Our analysis of the bolometric light curve indicates that only 0.14 ± 0.04 M ⊙ of 56Ni was synthesized in the explosion of SN 2016ije, which suggests a less energetic thermonuclear explosion when compared to normal SNe Ia, and this left a considerable amount of unburned materials in the ejecta. Spectroscopically, SN 2016ije resembles other SN 2002es-like SNe Ia, except that the ejecta velocity inferred from its carbon absorption line (∼4500 km s−1) is much lower than that from silicon lines (∼8300 km s−1) at around the maximum light. Additionally, most of the absorption lines are broader than other 02es-like SNe Ia. These peculiarities suggest the presence of significant unburned carbon in the inner region and a wide line-forming region along the line of sight. These characteristics suggest that SN 2016ije might originate from the violent merger of a white dwarf binary system, when viewed near an orientation along the iron-group-element cavity caused by the companion star. [ABSTRACT FROM AUTHOR]

We report the discovery of a transient seen in a strongly lensed arc at redshift z s = 1.2567 in Hubble Space Telescope imaging of the Abell 370 galaxy cluster. The transient is detected at 29.51 ± 0.14 AB mag in a WFC3/UVIS F200LP difference image made using observations from two different epochs, obtained in the framework of the Flashlights programme, and is also visible in the F350LP band (m F350LP ≈ 30.53 ± 0.76 AB mag). The transient is observed on the negative-parity side of the critical curve at a distance of ∼0.6 arcsec from it, greater than previous examples of lensed stars. The large distance from the critical curve yields a significantly smaller macromagnification, but our simulations show that bright, O/B-type supergiants can reach sufficiently high magnifications to be seen at the observed position and magnitude. In addition, the observed transient image is a trailing image with an observer-frame time delay of ∼+0.8 d from its expected counterpart, so that any transient lasting for longer than that should have also been seen on the minima side and is thus excluded. This, together with the blue colour we measure for the transient (m F200LP − m F350LP ≈ [−0.3, −1.6] AB), rules out most other transient candidates such as (kilo)novae, for example, and makes a lensed star the prime candidate. Assuming that the transient is indeed a lensed star as suggested, many more such events should be detected in the near future in cluster surveys with the Hubble Space Telescope and JWST. [ABSTRACT FROM AUTHOR]

We analyse photometric observations of the supernova (SN) impostor SN 2000ch in NGC 3432 covering the time since its discovery. This source was previously observed to have four outbursts in 2000–2010. Observations now reveal at least three additional outbursts in 2004–2007, and 16 outbursts in 2010–2022. Outburst light curves are irregular and multipeaked, exhibiting a wide variety of peak magnitude, duration, and shape. The outbursts after 2008 repeat with a period of 200.7 ± 2 d, while the outburst in 2000 seems to match with a shorter period. The next outburst should occur around January/February 2023. We propose that these periodic eruptions arise from violent interaction around times of periastron in an eccentric binary system, similar to the periastron encounters of η Carinae leading up to its Great Eruption, and resembling the erratic pre-SN eruptions of SN 2009ip. We attribute the irregularity of the eruptions to the interplay between the orbit and the variability of the luminous blue variable (LBV) primary star, wherein each successive periastron pass may have a different intensity or duration due to the changing radius and mass-loss rate of the LBV-like primary. Such outbursts may occasionally be weak or undetectable if the LBV is relatively quiescent at periastron but can be much more extreme when the LBV is active. The observed change in orbital period may be a consequence of mass lost in outbursts. Given the similarity to the progenitor of SN 2009ip, SN 2000ch deserves continued attention in the event it is headed for a stellar merger or an SN-like explosion. [ABSTRACT FROM AUTHOR]

We combine our dynamical modeling black-hole mass measurements from the Lick AGN Monitoring Project 2016 sample with measured cross-correlation time lags and line widths to recover individual scale factors, f, used in traditional reverberation-mapping analyses. We extend our sample by including prior results from Code for AGN Reverberation and Modeling of Emission Lines (caramel) studies that have utilized our methods. Aiming to improve the precision of black-hole mass estimates, as well as uncover any regularities in the behavior of the broad-line region (BLR), we search for correlations between f and other AGN/BLR parameters. We find (i) evidence for a correlation between the virial coefficient log 10 (f mean , σ) and black-hole mass, (ii) marginal evidence for a similar correlation between log 10 (f rms , σ) and black-hole mass, (iii) marginal evidence for an anticorrelation of BLR disk thickness with log 10 (f mean , FWHM) and log 10 (f rms , FWHM) , and (iv) marginal evidence for an anticorrelation of inclination angle with log 10 (f mean , FWHM) , log 10 (f rms , σ) , and log 10 (f mean , σ). Last, we find marginal evidence for a correlation between line-profile shape, when using the root-mean-square spectrum, log 10 (FWHM / σ) rms , and the virial coefficient, log 10 (f rms , σ) , and investigate how BLR properties might be related to line-profile shape using caramel models. [ABSTRACT FROM AUTHOR]

In late 2014, four images of supernova (SN) "Refsdal," the first known example of a strongly lensed SN with multiple resolved images, were detected in the MACS J1149 galaxy-cluster field. Following the images' discovery, the SN was predicted to reappear within hundreds of days at a new position ∼8″ away in the field. The observed reappearance in late 2015 makes it possible to carry out Refsdal's original proposal to use a multiply imaged SN to measure the Hubble constant H 0, since the time delay between appearances should vary inversely with H 0. Moreover, the position, brightness, and timing of the reappearance enable a novel test of the blind predictions of galaxy-cluster models, which are typically constrained only by the positions of multiply imaged galaxies. We have developed a new photometry pipeline that uses DOLPHOT to measure the fluxes of the five images of SN Refsdal from difference images. We apply four separate techniques to perform a blind measurement of the relative time delays and magnification ratios between the last image SX and the earlier images S1–S4. We measure the relative time delay of SX–S1 to be 376.0 − 5.5 + 5.6 days and the relative magnification to be 0.30 − 0.3 + 0.5 . This corresponds to a 1.5% precision on the time delay and 17% precision for the magnification ratios and includes uncertainties due to millilensing and microlensing. In an accompanying paper, we place initial and blind constraints on the value of the Hubble constant. [ABSTRACT FROM AUTHOR]

Ultraviolet light from early galaxies is thought to have ionized gas in the intergalactic medium. However, there are few observational constraints on this epoch because of the faintness of those galaxies and the redshift of their optical light into the infrared. We report the observation, in JWST imaging, of a distant galaxy that is magnified by gravitational lensing. JWST spectroscopy of the galaxy, at rest-frame optical wavelengths, detects strong nebular emission lines that are attributable to oxygen and hydrogen. The measured redshift is z = 9.51 ± 0.01, corresponding to 510 million years after the Big Bang. The galaxy has a radius of 16:2þ4:6 -7:2 parsecs, which is substantially more compact than galaxies with equivalent luminosity at z ~ 6 to 8, leading to a high star formation rate surface density. [ABSTRACT FROM AUTHOR]

We present extensive optical/ultraviolet observations and modelling analysis for the nearby SN 1987A-like peculiar Type II supernova (SN) 2018hna. Both photometry and spectroscopy covered phases extending to >500 d after the explosion, making it one of the best-observed SN II of this subtype. SN 2018hna is obviously bluer than SN 1987A during the photospheric phase, suggesting higher photospheric temperature, which may account for weaker Ba  ii  λ6142 lines in its spectra. Analysis of early-time temperature evolution suggests a radius of ∼45 R⊙ for the progenitor of SN 2018hna, consistent with a blue supergiant (BSG). By fitting the bolometric light curve with hydrodynamical models, we find that SN 2018hna has an ejecta mass of ∼(13.7–17.7) M⊙, a kinetic energy of ∼(1.0–1.2) × 1051 erg, and a 56Ni mass of about 0.05 M⊙. Moreover, based on standard stellar evolution and the oxygen mass (0.44–0.73 M⊙) deduced from nebular [O  i ] lines, the progenitor of SN 2018hna is expected to have an initial main-sequence mass <16 M⊙. In principle, such a relatively low-mass star cannot end as a BSG just before core-collapse, except some unique mechanisms are involved, such as rapid rotation, restricted semiconvection, etc. On the other hand, binary scenario may be more favourable, like in the case of SN 1987A. While the much lower oxygen mass inferred for SN 2018hna may imply that its progenitor system also had much lower initial masses than that of SN 1987A. [ABSTRACT FROM AUTHOR]

Spectra of broad-lined Type Ic supernovae (SNe Ic-BL), the only kind of SN observed at the locations of long-duration gamma-ray bursts (LGRBs), exhibit wide features indicative of high ejecta velocities (∼0.1c). We study the host galaxies of a sample of 245 low-redshift (z < 0.2) core-collapse SNe, including 17 SNe Ic-BL, discovered by galaxy-untargeted searches, and 15 optically luminous and dust-obscured z < 1.2 LGRBs. We show that, in comparison with Sloan Digital Sky Survey galaxies having similar stellar masses, the hosts of low-redshift SNe Ic-BL and z < 1.2 LGRBs have high stellar mass and star formation rate densities. Core-collapse SNe having typical ejecta velocities, in contrast, show no preference for such galaxies. Moreover, we find that the hosts of SNe Ic-BL, unlike those of SNe Ib/Ic and SNe II, exhibit high gas velocity dispersions for their stellar masses. The patterns likely reflect variations among star-forming environments and suggest that LGRBs can be used as probes of conditions in high-redshift galaxies. They may be caused by efficient formation of massive binary progenitor systems in densely star-forming regions, or, less probably, a higher fraction of stars created with the initial masses required for an SN Ic-BL or LGRB. Finally, we show that the preference of SNe Ic-BL and LGRBs for galaxies with high stellar mass and star formation rate densities cannot be attributed to a preference for low metal abundances but must reflect the influence of a separate environmental factor. [ABSTRACT FROM AUTHOR]

Presents a summary of a study on the presence of black holes in the Milky Way Galaxy presented at the tenth annual symposium on Frontiers of Science held from November 19-21, 1998 at the Arnold and Mabel Beckman Center of the National Academies of Sciences and Engineering in Irvine, California.

Optical spectropolarimetry of the normal thermonuclear supernova (SN) 2019np from −14.5 to +14.5 d relative to B -band maximum detected an intrinsic continuum polarization (p cont) of 0.21 ± 0.09 per cent at the first epoch. Between days −11.5 and  +0.5, p cont remained ∼0 and by day +14.5 was again significant at 0.19 ± 0.10 per cent. Not considering the first epoch, the dominant axis of |${\rm Si\, {\small II}}$|  λ6355 was roughly constant staying close the continuum until both rotated in opposite directions on day  +14.5. Detailed radiation-hydrodynamical simulations produce a very steep density slope in the outermost ejecta so that the low first-epoch p cont ≈ 0.2 per cent nevertheless suggests a separate structure with an axis ratio ∼2 in the outer carbon-rich (3.5–4) × 10−3 M⊙. Large-amplitude fluctuations in the polarization profiles and a flocculent appearance of the polar diagram for the |${\rm Ca\, {\small II}}$|  near-infrared triplet (NIR3) may be related by a common origin. The temporal evolution of the polarization spectra agrees with an off-centre delayed detonation. The late-time increase in polarization and the possible change in position angle are also consistent with an aspherical 56Ni core. The p cont and the absorptions due to |${\rm Si\, {\small II}}$| λ6355 and |${\rm Ca\, {\small II}}$|  NIR3 form in the same region of the extended photosphere, with an interplay between line occultation and thermalization producing p. Small-scale polarization features may be due to small-scale structures, but many could be related to atomic patterns of the quasi-continuum; they hardly have an equivalent in the total-flux spectra. We compare SN 2019np to other SNe and develop future objectives and strategies for SN Ia spectropolarimetry. [ABSTRACT FROM AUTHOR]

A rare class of supernovae (SNe) is characterized by strong interaction between the ejecta and several solar masses of circumstellar matter (CSM) as evidenced by strong Balmer-line emission. Within the first few weeks after the explosion, they may display spectral features similar to overluminous Type Ia SNe, while at later phase their observation properties exhibit remarkable similarities with some extreme case of Type IIn SNe that show strong Balmer lines years after the explosion. We present polarimetric observations of SN 2018evt obtained by the ESO Very Large Telescope from 172 to 219 d after the estimated time of peak luminosity to study the geometry of the CSM. The non-zero continuum polarization decreases over time, suggesting that the mass-loss of the progenitor star is aspherical. The prominent H α emission can be decomposed into a broad, time-evolving component and an intermediate-width, static component. The former shows polarized signals, and it is likely to arise from a cold dense shell (CDS) within the region between the forward and reverse shocks. The latter is significantly unpolarized, and it is likely to arise from shocked, fragmented gas clouds in the H-rich CSM. We infer that SN 2018evt exploded inside a massive and aspherical circumstellar cloud. The symmetry axes of the CSM and the SN appear to be similar. SN 2018evt shows observational properties common to events that display strong interaction between the ejecta and CSM, implying that they share similar circumstellar configurations. Our preliminary estimate also suggests that the circumstellar environment of SN 2018evt has been significantly enriched at a rate of ∼0.1 M⊙ yr−1 over a period of >100 yr. [ABSTRACT FROM AUTHOR]

As part of a larger completed Hubble Space Telescope (HST) Snapshot program, we observed the sites of six nearby core-collapse supernovae (SNe) at high spatial resolution: SN 2012A, SN 2013ej, SN 2016gkg, SN 2017eaw, SN 2018zd, and SN 2018aoq. These observations were all conducted at sufficiently late times in each SN's evolution to demonstrate that the massive-star progenitor candidate identified in each case in pre-explosion imaging data had indeed vanished and was therefore most likely the actual progenitor. However, we have determined for SN 2016gkg that the progenitor candidate was most likely a blend of two objects: the progenitor, which itself has likely vanished, and another closely neighbouring star. We thus provide a revised estimate of that progenitor's properties: a binary system with a hydrogen-stripped primary star at explosion with effective temperature ≈6300–7900 K, bolometric luminosity ≈104.65 L⊙, radius ≈118–154 R⊙, and initial mass 9.5–11 M⊙. Utilizing late-time additional archival HST data nearly contemporaneous with our Snapshots, we also show that SN 2017eaw had a luminous ultraviolet excess, which is best explained as a result of ongoing interaction of the SN shock with pre-existing circumstellar matter. We offer the caveat, particularly in the case of SN 2013ej, that obscuration from SN dust may be compromising our conclusions. This sample adds to the growing list of confirmed or likely core-collapse SN progenitors. [ABSTRACT FROM AUTHOR]

In this work, photometric and spectroscopic analyses of a very low-luminosity Type IIb supernova (SN) 2016iyc have been performed. SN 2016iyc lies near the faint end among the distribution of similar supernovae (SNe). Given lower ejecta mass (M ej) and low nickel mass (M Ni) from the literature, combined with SN 2016iyc lying near the faint end, one-dimensional stellar evolution models of 9–14 M⊙ zero-age main-sequence (ZAMS) stars as the possible progenitors of SN 2016iyc have been performed using the publicly available code mesa. Moreover, synthetic explosions of the progenitor models have been simulated, using the hydrodynamic evolution codes stella and snec. The bolometric luminosity light curve and photospheric velocities produced through synthetic explosions of ZAMS stars of mass in the range of 12–13 M⊙ having a pre-supernova radius R 0 = (204–300) R⊙, with M ej = (1.89–1.93) M⊙, explosion energy E exp = (0.28–0.35) × 1051 erg, and M Ni < 0.09 M⊙, are in good agreement with observations; thus, SN 2016iyc probably exploded from a progenitor near the lower mass limits for SNe IIb. Finally, hydrodynamic simulations of the explosions of SN 2016gkg and SN 2011fu have also been performed to compare intermediate- and high-luminosity examples among well-studied SNe IIb. The results of progenitor modelling and synthetic explosions for SN 2016iyc, SN 2016gkg, and SN 2011fu exhibit a diverse range of mass for the possible progenitors of SNe IIb. [ABSTRACT FROM AUTHOR]

The abundance distribution in the ejecta of the peculiar slowly declining Type Ia supernova (SN Ia) SN 1999aa is obtained by modelling a time series of optical spectra. Similar to SN 1991T, SN 1999aa was characterized by early-time spectra dominated by Fe  iii features and a weak Si  ii  6355 Å line, but it exhibited a high-velocity Ca  ii  H&K line and morphed into a spectroscopically normal SN Ia earlier. Three explosion models are investigated, yielding comparable fits. The innermost layers are dominated by ∼0.3 M⊙ of neutron-rich stable iron-group elements, mostly stable iron. Above that central region lies a 56Ni-dominated shell, extending to |$v \approx 11\, 000$| – |$12\, 000$| km s−1, with mass ∼0.65 M⊙. These inner layers are therefore similar to those of normal SNe Ia. However, the outer layers exhibit composition peculiarities similar to those of SN 1991T: The intermediate-mass elements shell is very thin, containing only ∼0.2 M⊙, and is sharply separated from an outer oxygen-dominated shell, which includes ∼0.22 M⊙. These results imply that burning suddenly stopped in SN 1999aa. This is a feature SN 1999aa shares with SN 1991T, and explains the peculiarities of both SNe, which are quite similar in nature apart from the different luminosities. The spectroscopic path from normal to SN 1991T-like SNe Ia cannot be explained solely by a temperature sequence. It also involves composition layering differences, suggesting variations in the progenitor density structure or in the explosion parameters. [ABSTRACT FROM AUTHOR]

We present optical spectropolarimetry of the tidal disruption event (TDE) AT 2019qiz on days +0 and +29 relative to maximum brightness. Continuum polarization, which informs the shape of the electron-scattering surface, was found to be consistent with 0 per cent at peak brightness. On day +29, the continuum polarization rose to ∼1 per cent, making this the first reported spectropolarimetric evolution of a TDE. These findings are incompatible with a naked eccentric disc that lacks significant mass outflow. Instead, the spectropolarimetry paints a picture wherein, at maximum brightness, high-frequency emission from the accretion disc is reprocessed into the optical band by a nearly spherical, optically thick, electron-scattering photosphere located far away from the black hole. We estimate the radius of the scattering photosphere to be |$\sim 100\rm \, au$| at maximum brightness – significantly larger than the tidal radius (⁠|$\sim 1\rm \, au$|⁠) and the thermalization radius (⁠|$\sim 30\rm \, au$|⁠) where the optical continuum is formed. A month later, as the fallback rate drops and the scattering photosphere recedes, the continuum polarization increases, revealing a moderately aspherical interior. We also see evidence for smaller scale density variations in the scattering photosphere, inferred from the scatter of the data in the Stokes q − u plane. On day +29, the H α emission-line peak is depolarized to ∼0.3 per cent (compared to ∼1 per cent continuum polarization), and displays a gradual rise towards the line's redder wavelengths. This observation indicates the H α line formed near the electron-scattering radius. [ABSTRACT FROM AUTHOR]