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The host galaxies of tidal disruption events (TDEs) have been shown to possess peculiar properties, including high central light concentrations, unusual star formation histories, and "green" colors. The ubiquity of these large-scale galaxy characteristics among TDE host populations suggests that they may serve to boost the TDE rate in such galaxies by influencing the nuclear stellar dynamics. We present the first population study of integral field spectroscopy for 13 TDE host galaxies across all spectral classes and X-ray brightnesses with the purpose of investigating their large-scale properties. We derive the black hole masses via stellar kinematics (i.e., the M – σ relation) and find masses in the range 5.0 ≲ log (M BH / M ⊙) ≲ 8.0 , with a distribution dominated by black holes with M BH ∼ 106 M ⊙. We find one object with M BH ≳ 108 M ⊙, above the "Hills mass", which if the disrupted star was of solar type, allows a lower limit of a ≳ 0.16 to be placed on its spin, lending further support to the proposed connection between featureless TDEs and jetted TDEs. We also explore the level of rotational support in the TDE hosts, quantified by (V / σ) e , a parameter that has been shown to correlate with the stellar age and may explain the peculiar host-galaxy preferences of TDEs. We find that the TDE hosts exhibit a broad range in (V / σ) e following a similar distribution as E + A galaxies, which have been shown to be overrepresented among TDE host populations. [ABSTRACT FROM AUTHOR]

We present multiwavelength modeling of the afterglow from the long γ-ray burst (GRB) 160625B using Markov Chain Monte Carlo techniques of the afterglowpy Python package. GRB 160625B is an extremely bright burst with a rich set of observations spanning from radio to γ-ray frequencies. These observations range from ∼0.1 days to >1000 days, thus making this event extremely well suited to such modeling. In this work we compare top-hat and Gaussian jet structure types in order to find best-fit values for the GRB jet collimation angle, viewing angle, and other physical parameters. We find that a Gaussian-shaped jet is preferred (2.7σ–5.3σ) over the traditional top-hat model. Our estimate for the opening angle of the burst ranges from 1.°26 to 3.°90, depending on jet-shape model. We also discuss the implications that assumptions on jet shape, viewing angle, and particularly the participation a fraction of electrons have on the final estimation of GRB intrinsic energy release and the resulting energy budget of the relativistic outflow. Most notably, allowing the participation fraction to vary results in an estimated total relativistic energy of ∼1053 erg. This is two orders of magnitude higher than when the total fraction is assumed to be unity; thus, this parameter has strong relevance for placing constraints on long GRB central engines, details of the circumburst media, and host environment. [ABSTRACT FROM AUTHOR]

The detection of gravitational waves from the binary neuron star merger GW170817 and electromagnetic counterparts GRB170817A and AT2017gfo kick-started the field of gravitational-wave multimessenger astronomy. The optically red to near-infrared emission ("red" component) of AT2017gfo was readily explained as produced by the decay of newly created nuclei produced by rapid neutron capture (a kilonova). However, the ultraviolet to optically blue emission ("blue" component) that was dominant at early times (up to 1.5 days) received no consensus regarding its driving physics. Among many explanations, two leading contenders are kilonova radiation from a lanthanide-poor ejecta component and shock interaction (cocoon emission). In this work, we simulate AT2017gfo-like light curves and perform a Bayesian analysis to study whether an ultraviolet satellite capable of rapid gravitational-wave follow-up, could distinguish between physical processes driving the early "blue" component. We find that ultraviolet data starting at 1.2 hr distinguishes the two early radiation models up to 160 Mpc, implying that an ultraviolet mission like Dorado would significantly contribute to insights into the driving emission physics of the postmerger system. While the same ultraviolet data and optical data starting at 12 hr have limited ability to constrain model parameters separately, the combination of the two unlocks tight constraints for all but one parameter of the kilonova model up to 160 Mpc. We further find that a Dorado-like ultraviolet satellite can distinguish the early radiation models up to at least 130 (60) Mpc if data collection starts within 3.2 (5.2) hr for AT2017gfo-like light curves. [ABSTRACT FROM AUTHOR]

ABSTRACT We present the discovery and characterization of PTF10iya, a short-lived (Δ t≈ 10 d, with an optical decay rate of ∼0.3 mag d−1), luminous ( mag) transient source found by the Palomar Transient Factory. The ultraviolet/optical spectral energy distribution is reasonably well fitted by a blackbody with T≈ (1-2) × 104 K and peak bolometric luminosity LBB≈ (1-5) × 1044 erg s−1 (depending on the details of the extinction correction). A comparable amount of energy is radiated in the X-ray band that appears to result from a distinct physical process. The location of PTF10iya is consistent with the nucleus of a star-forming galaxy ( z= 0.224 05 ± 0.000 06) to within 350 mas (99.7 per cent confidence radius), or a projected distance of less than 1.2 kpc. At first glance, these properties appear reminiscent of the characteristic 'big blue bump' seen in the near-ultraviolet spectra of many active galactic nuclei (AGNs). However, emission-line diagnostics of the host galaxy, along with a historical light curve extending back to 2007, show no evidence for AGN-like activity. We therefore consider whether the tidal disruption of a star by an otherwise quiescent supermassive black hole may account for our observations. Though with limited temporal information, PTF10iya appears broadly consistent with the predictions for the early 'super-Eddington' phase of a solar-type star being disrupted by a ∼107 M⊙ black hole. Regardless of the precise physical origin of the accreting material, the large luminosity and short duration suggest that otherwise quiescent galaxies can transition extremely rapidly to radiate near the Eddington limit; many such outbursts may have been missed by previous surveys lacking sufficient cadence. [ABSTRACT FROM AUTHOR]

ABSTRACT We present photometry and spectroscopy of the peculiar Type II supernova (SN) SN 2010jp, also named PTF10aaxi. The light curve exhibits a linear decline with a relatively low peak absolute magnitude of only −15.9 (unfiltered), and a low radioactive decay luminosity at late times, which suggests a low synthesized nickel mass of M (56 Ni) ≲ 0.003 M⊙. Spectra of SN 2010jp display an unprecedented triple-peaked Hα line profile, showing (1) a narrow (full width at half-maximum > rsim800 km s −1) central component that suggests shock interaction with dense circumstellar material (CSM); (2) high-velocity blue and red emission features centred at −12 600 and +15 400 km s −1, respectively; and (3) very broad wings extending from −22 000 to +25 000 km s −1. These features persist over multiple epochs during the ∼100 d after explosion. We propose that this line profile indicates a bipolar jet-driven explosion, with the central component produced by normal SN ejecta and CSM interaction at mid and low latitudes, while the high-velocity bumps and broad-line wings arise in a non-relativistic bipolar jet. Two variations of the jet interpretation seem plausible: (1) a fast jet mixes 56Ni to high velocities in polar zones of the H-rich envelope; or (2) the reverse shock in the jet produces blue and red bumps in Balmer lines when a jet interacts with dense CSM. Jet-driven Type II SNe are predicted for collapsars resulting from a wide range of initial masses above 25 M⊙, especially at subsolar metallicity. This seems consistent with the SN host environment, which is either an extremely low-luminosity dwarf galaxy or the very remote parts of an interacting pair of star-forming galaxies. It also seems consistent with the apparently low 56Ni mass that may accompany black hole formation. We speculate that the jet survives to produce observable signatures because the star's H envelope was very low mass, having been mostly stripped away by the previous eruptive mass-loss indicated by the Type IIn features in the spectrum. [ABSTRACT FROM AUTHOR]

We present a systematic analysis of the X-ray emission of a sample of 17 optically selected, X-ray-detected tidal disruption events (TDEs) discovered between 2014 and 2021. The X-ray light curves show a diverse range of temporal behaviors, with most sources not following the expected power-law decline. The X-ray spectra are mostly extremely soft and consistent with thermal emission from the innermost region of an accretion disk, which cools as the accretion rate decreases. Three sources show formation of a hard X-ray corona at late times. The spectral energy distribution shape, probed by the ratio (L BB/ L X) between the UV/optical and X-ray, shows a wide range of L BB/ L X ∈ (0.5, 3000) at early times and converges to disklike values of L BB/ L X ∈ (0.5, 10) at late times. We estimate the fraction of optically discovered TDEs with L X ≥ 1042 erg s−1 to be at least 40% and show that X-ray loudness is independent of black hole mass. We argue that distinct disk formation timescales are unlikely to be able to explain the diverse range of X-ray evolution. We combine our sample with X-ray-discovered ones to construct an X-ray luminosity function, best fit by a broken power law, with a break at L X ≈ 1044 erg s−1. We show that there is no dichotomy between optically and X-ray-selected TDEs; instead, there is a continuum of early-time L BB/ L X, at least as wide as L BB/ L X ∈ (0.1, 3000), with optical/X-ray surveys selecting preferentially, but not exclusively, from the higher/lower end of the distribution. Our findings are consistent with unification models for the overall TDE population. [ABSTRACT FROM AUTHOR]

The impact of the Double Asteroid Redirection Test (DART) spacecraft with Dimorphos allows us to study asteroid collision physics, including momentum transfer, the ejecta properties, and the visibility of such events in the Solar system. We report observations of the DART impact in the ultraviolet (UV), visible light, and near-infrared (IR) wavelengths. The observations support the existence of at least two separate components of the ejecta: a fast and a slow component. The fast-ejecta component is composed of a gaseous phase, moving at about 1.6 km s−1 with a mass of ≲104 kg. The fast ejecta is detected in the UV and visible light, but not in the near-IR z -band observations. Fitting a simplified optical thickness model to these observations allows us to constrain some of the properties of the fast ejecta, including its scattering efficiency and the opacity of the gas. The slow ejecta component is moving at typical velocities of up to about 10 m s−1. It is composed of micrometer-size particles, that have a scattering efficiency, at the direction of the observer, of the order of 10−3 and a total mass of ∼106 kg. The larger particles in the slow ejecta, whose size is bound to be in the range between ∼1 mm and ∼1 m, likely have a scattering efficiency larger than that of the pre-impact Didymos system. [ABSTRACT FROM AUTHOR]

Detections of the tidal disruption flares (TDFs) of stars by supermassive black holes (SMBHs) are rapidly accumulating as optical surveys improve. These detections may provide constraints on SMBH demographics, stellar dynamics, and stellar evolution in galaxies. To maximize this scientific impact, we require a better understanding of how astrophysical parameters interact with survey selection effects in setting the properties of detected flares. We develop a framework for modeling the distributions of optical TDF detections in surveys across attributes of the host galaxies and the flares themselves. This model folds in effects of the stellar disruption rate in each galaxy, the flare luminosity and temperature distributions, the effects of obscuration and reddening by dust in the host galaxy, and survey selection criteria. We directly apply this model to the sample of TDFs detected by the Zwicky Transient Facility, and find that the overall flare detection rate is in line with simple theoretical expectation. The model can also reproduce the distribution of total stellar mass and redshift of the host galaxies, but fails to match all details of the detected flares, such as their luminosity and temperature distributions. We also find that dust obscuration likely plays an important role in suppressing the TDF detection rate in star-forming galaxies. While we do not find that the unusual preference of TDFs to have hosts in post-starburst galaxies in the "green valley" can be entirely explained by selection effects, our model can help to quantify the true rate enhancement in those galaxies. [ABSTRACT FROM AUTHOR]

We present post-jet-break Hubble Space Telescope, Very Large Array, and Chandra observations of the afterglow of the long γ-ray bursts GRB 160625B (between 69 and 209 days) and GRB 160509A (between 35 and 80 days). We calculate the post-jet-break decline rates of the light curves and find the afterglow of GRB 160625B is inconsistent with a simple t−3/4 steepening over the break, expected from the geometric effect of the jet edge entering our line of sight. However, the favored optical post-break decline () is also inconsistent with the fν ∝ t−p decline (where p ≈ 2.3 from the pre-break light curve), which is expected from exponential lateral expansion of the jet; perhaps suggesting lateral expansion that only affects a fraction of the jet. The post-break decline of GRB 160509A is consistent with both the t−3/4 steepening and with fν ∝ t−p. We also use boxfit to fit afterglow models to both light curves and find both to be energetically consistent with a millisecond magnetar central engine, but the magnetar parameters need to be extreme (i.e., E ∼ 3 × 1052 erg). Finally, the late-time radio light curves of both afterglows are not reproduced well by boxfit and are inconsistent with predictions from the standard jet model; instead, both are well represented by a single power-law decline (roughly fν ∝ t−1) with no breaks. This requires a highly chromatic jet break () and possibly a two-component jet for both bursts. [ABSTRACT FROM AUTHOR]

We present ZTF18aaqjovh (SN 2018bvw), a high-velocity ("broad-lined") stripped-envelope (Type Ic) supernova (Ic-BL SN) discovered in the Zwicky Transient Facility one-day cadence survey. ZTF18aaqjovh shares a number of features in common with engine-driven explosions: the photospheric velocity and the shape of the optical light curve are very similar to those of the Type Ic-BL SN 1998bw, which was associated with a low-luminosity gamma-ray burst (LLGRB) and had relativistic ejecta. However, the radio luminosity of ZTF18aaqjovh is almost two orders of magnitude fainter than that of SN 1998bw at the same velocity phase, and the shock velocity is at most mildly relativistic (v = 0.06–0.4c). A search of high-energy catalogs reveals no compelling gamma-ray burst (GRB) counterpart to ZTF18aaqjovh, and the limit on the prompt GRB luminosity of excludes a classical GRB but not an LLGRB. Altogether, ZTF18aaqjovh represents another transition event between engine-driven SNe associated with GRBs and "ordinary" Ic-BL SNe. [ABSTRACT FROM AUTHOR]

Tidal disruption events (TDEs) offer a unique way to study dormant black holes. While the number of observed TDEs has grown thanks to the emergence of wide-field surveys in the past few decades, questions regarding the nature of the observed optical, UV, and X-ray emission remain. We present a uniformly selected sample of 30 spectroscopically classified TDEs from the Zwicky Transient Facility Phase I survey operations with follow-up Swift UV and X-ray observations. Through our investigation into correlations between light-curve properties, we recover a shallow positive correlation between the peak bolometric luminosity and decay timescales. We introduce a new spectroscopic class of TDE, TDE-featureless, which are characterized by featureless optical spectra. The new TDE-featureless class shows larger peak bolometric luminosities, peak blackbody temperatures, and peak blackbody radii. We examine the differences between the X-ray bright and X-ray faint populations of TDEs in this sample, finding that X-ray bright TDEs show higher peak blackbody luminosities than the X-ray faint subsample. This sample of optically selected TDEs is the largest sample of TDEs from a single survey yet, and the systematic discovery, classification, and follow-up of this sample allows for robust characterization of TDE properties, an important stepping stone looking forward toward the Rubin era. [ABSTRACT FROM AUTHOR]

We report on the direct detection and characterization of the probable red supergiant (RSG) progenitor of the intermediate-luminosity Type II-Plateau (II-P) supernova (SN) 2012aw in the nearby (10.0 Mpc) spiral galaxy Messier 95 (M95; NGC 3351). We have identified the star in both Hubble Space Telescope images of the host galaxy, obtained 17-18 yr prior to the explosion, and near-infrared ground-based images, obtained 6-12 yr prior to the SN. The luminous supergiant showed evidence for substantial circumstellar dust, manifested as excess line-of-sight extinction. The effective total-to-selective ratio of extinction to the star was R′V ≈ 4.35, which is significantly different from that of diffuse interstellar dust (i.e., RV = 3.1), and the total extinction to the star was therefore, on average, AV ≈ 3.1 mag. We find that the observed spectral energy distribution for the progenitor star is consistent with an effective temperature of 3600 K (spectral type M3), and that the star therefore had a bolometric magnitude of –8.29. Through comparison with recent theoretical massive-star evolutionary tracks we can infer that the RSG progenitor had an initial mass 15 ≲ Mini(M☼) < 20. Interpolating by eye between the available tracks, we surmise that the star had initial mass ∼17-18 M☼. The circumstellar dust around the progenitor must have been destroyed in the explosion, as the visual extinction to the SN is found to be low (AV = 0.24 mag with RV = 3.1). [ABSTRACT FROM AUTHOR]

We present X-ray, UV, optical, and radio observations of the nearby (≈78 Mpc) tidal disruption event AT2021ehb/ZTF21aanxhjv during its first 430 days of evolution. AT2021ehb occurs in the nucleus of a galaxy hosting a≈107 M ⊙ black hole (M BH inferred from host galaxy scaling relations). High-cadence Swift and Neutron Star Interior Composition Explorer (NICER) monitoring reveals a delayed X-ray brightening. The spectrum first undergoes a gradual soft â†' hard transition and then suddenly turns soft again within 3 days at δ t ≈272 days during which the X-ray flux drops by a factor of 10. In the joint NICER+NuSTAR observation (δ t = 264 days, harder state), we observe a prominent nonthermal component up to 30 keV and an extremely broad emission line in the iron K band. The bolometric luminosity of AT2021ehb reaches a maximum of 6.0 âˆ' 3.8 + 10.4 % L Edd when the X-ray spectrum is the hardest. During the dramatic X-ray evolution, no radio emission is detected, the UV/optical luminosity stays relatively constant, and the optical spectra are featureless. We propose the following interpretations: (i) the soft â†' hard transition may be caused by the gradual formation of a magnetically dominated corona; (ii) hard X-ray photons escape from the system along solid angles with low scattering optical depth (∼a few) whereas the UV/optical emission is likely generated by reprocessing materials with much larger column densityâ€"the system is highly aspherical; and (iii) the abrupt X-ray flux drop may be triggered by the thermalâ€"viscous instability in the inner accretion flow, leading to a much thinner disk. [ABSTRACT FROM AUTHOR]

The nearby Type II active galactic nucleus (AGN) 1ES 1927+654 went through a violent changing-look (CL) event beginning 2017 December during which the optical and UV fluxes increased by four magnitudes over a few months, and broad emission lines newly appeared in the optical/UV. By 2018 July, the X-ray coronal emission had completely vanished, only to reappear a few months later. In this work we report the evolution of the radio, optical, UV and X-rays from the preflare state through mid-2021 with new and archival data from the Very Long Baseline Array, the European VLBI Network, the Very Large Array, the Telescopio Nazionale Galileo, Gran Telescopio Canarias, The Neil Gehrels Swift observatory, and XMM-Newton. The main results from our work are (i) the source has returned to its pre-CL state in optical, UV, and X-ray; the diskâ€"corona relation has been reestablished as it has been in the pre-CL state, with an α OX ∼ 1.02. The optical spectra are dominated by narrow emission lines. (ii) The UV light curve follows a shallower slope of ∝ t âˆ'0.91±0.04 compared to that predicted by a tidal disruption event. We conjecture that a magnetic flux inversion event is the possible cause for this enigmatic event. (iii) The compact radio emission which we tracked in the pre-CL (2014), during CL (2018), and post-CL (2021) at spatial scales <1 pc was at its lowest level during the CL event in 2018, nearly contemporaneous with a low 2â€"10 keV emission. The radio to X-ray ratio of the compact source L Radio/ L Xâˆ'ray ∼ 10âˆ'5.5 follows the GĂĽdelâ€"Benz relation, typically found in coronally active stars and several AGNs. (iv) We do not detect any presence of nascent jets at the spatial scales of ∼5â€"10 pc. [ABSTRACT FROM AUTHOR]

The observed durations of prompt gamma-ray emission from gamma-ray bursts (GRBs) are often used to infer the progenitors and energetics of the sources. Inaccurate duration measurements will have a significant impact on constraining the processes powering the bursts. The “tip-of-the-iceberg” effect describes how the observed signal is lost into background noise; lower instrument sensitivity leads to higher measurement bias. In this study, we investigate how observing conditions, such as the number of enabled detectors, background level, and incident angle of the source relative to the detector plane, affect the measured duration of GRB prompt emission observed with the Burst Alert Telescope on board the Neil Gehrels Swift Observatory (Swift/BAT). We generate “simple-pulse” light curves from an analytical fast rise exponential decay function and from a sample of eight real GRB light curves. We fold these through the Swift/BAT instrument response function to simulate light curves Swift/BAT would have observed for specific observing conditions. We find duration measurements are highly sensitive to observing conditions and the incident angle of the source has the highest impact on measurement bias. In most cases duration measurements of synthetic light curves are significantly shorter than the true burst duration. For the majority of our sample, the percentage of duration measurements consistent with the true duration is as low as ∼25%â€"45%. In this article, we provide quantification of the tip-of-the-iceberg effect on GRB light curves due to Swift/BAT instrumental effects for several unique light curves. [ABSTRACT FROM AUTHOR]

Type Ia supernovae have been used empirically as 'standard candles' to demonstrate the acceleration of the expansion of the Universe even though fundamental details, such as the nature of their progenitor systems and how the stars explode, remain a mystery. There is consensus that a white dwarf star explodes after accreting matter in a binary system, but the secondary body could be anything from a main-sequence star to a red giant, or even another white dwarf. This uncertainty stems from the fact that no recent type Ia supernova has been discovered close enough to Earth to detect the stars before explosion. Here we report early observations of supernova SN 2011fe in the galaxy M101 at a distance from Earth of 6.4 megaparsecs. We find that the exploding star was probably a carbon-oxygen white dwarf, and from the lack of an early shock we conclude that the companion was probably a main-sequence star. Early spectroscopy shows high-velocity oxygen that slows rapidly, on a timescale of hours, and extensive mixing of newly synthesized intermediate-mass elements in the outermost layers of the supernova. A companion paper uses pre-explosion images to rule out luminous red giants and most helium stars as companions to the progenitor. [ABSTRACT FROM AUTHOR]

Spectroscopic detection of narrow emission lines traces the presence of circumstellar mass distributions around massive stars exploding as core-collapse supernovae. Transient emission lines disappearing shortly after the supernova explosion suggest that the material spatial extent is compact and implies an increased mass loss shortly prior to explosion. Here, we present a systematic survey for such transient emission lines (Flash Spectroscopy) among Type II supernovae detected in the first year of the Zwicky Transient Facility survey. We find that at least six out of ten events for which a spectrum was obtained within two days of the estimated explosion time show evidence for such transient flash lines. Our measured flash event fraction (>30% at 95% confidence level) indicates that elevated mass loss is a common process occurring in massive stars that are about to explode as supernovae. [ABSTRACT FROM AUTHOR]

While tidal disruption events (TDEs) have long been heralded as laboratories for the study of quiescent black holes, the small number of known TDEs and uncertainties in their emission mechanism have hindered progress toward this promise. Here we present 17 new TDEs that have been detected recently by the Zwicky Transient Facility along with Swift UV and X-ray follow-up observations. Our homogeneous analysis of the optical/UV light curves, including 22 previously known TDEs from the literature, reveals a clean separation of light-curve properties with spectroscopic class. The TDEs with Bowen fluorescence features in their optical spectra have smaller blackbody radii, lower optical luminosities, and higher disruption rates compared to the rest of the sample. The small subset of TDEs that show only helium emission lines in their spectra have the longest rise times, the highest luminosities, and the lowest rates. A high detection rate of Bowen lines in TDEs with small photometric radii could be explained by the high density that is required for this fluorescence mechanism. The stellar debris can provide a source for this dense material. Diffusion of photons through this debris may explain why the rise and fade timescale of the TDEs in our sample are not correlated. We also report, for the first time, the detection of soft X-ray flares from a TDE on ∼day timescales. Based on the fact that the X-ray flares peak at a luminosity similar to the optical/UV blackbody luminosity, we attribute them to brief glimpses through a reprocessing layer that otherwise obscures the inner accretion flow. [ABSTRACT FROM AUTHOR]

We present ZTF20aajnksq (AT 2020blt), a fast-fading (Δr = 2.3 mag in Δt = 1.3 days) red (g − r ≈ 0.6 mag) and luminous (M1626 Å = −25.9 mag) optical transient at z = 2.9 discovered by the Zwicky Transient Facility (ZTF). AT 2020blt shares several features in common with afterglows to long-duration gamma-ray bursts (GRBs): (1) an optical light curve well-described by a broken power law with a break at tj = 1 d (observer frame); (2) a luminous (L0.3–10 KeV = 1046 erg s−1) X-ray counterpart; and (3) luminous (L10 GHz = 4 × 1031 erg s−1 Hz−1) radio emission. However, no GRB was detected in the 0.74 days between the last ZTF nondetection (r > 21.36 mag) and the first ZTF detection (r = 19.60 mag), with an upper limit on the isotropic-equivalent gamma-ray energy release of Eγ,iso < 7 × 1052 erg. AT 2020blt is thus the third afterglow-like transient discovered without a detected GRB counterpart (after PTF11agg and ZTF19abvizsw) and the second (after ZTF19abvizsw) with a redshift measurement. We conclude that the properties of AT 2020blt are consistent with a classical (initial Lorentz factor Γ0 ≳ 100) on-axis GRB that was missed by high-energy satellites. Furthermore, by estimating the rate of transients with light curves similar to that of AT 2020blt in ZTF high-cadence data, we agree with previous results that there is no evidence for an afterglow-like phenomenon that is significantly more common than classical GRBs, such as dirty fireballs. We conclude by discussing the status and future of fast-transient searches in wide-field high-cadence optical surveys. [ABSTRACT FROM AUTHOR]

We present a systematic search for optical counterparts to 13 gravitational wave (GW) triggers involving at least one neutron star during LIGO/Virgo's third observing run (O3). We searched binary neutron star (BNS) and neutron star black hole (NSBH) merger localizations with the Zwicky Transient Facility (ZTF) and undertook follow-up with the Global Relay of Observatories Watching Transients Happen (GROWTH) collaboration. The GW triggers had a median localization area of 4480 deg2, a median distance of 267 Mpc, and false-alarm rates ranging from 1.5 to 10−25 yr−1. The ZTF coverage in the g and r bands had a median enclosed probability of 39%, median depth of 20.8 mag, and median time lag between merger and the start of observations of 1.5 hr. The O3 follow-up by the GROWTH team comprised 340 UltraViolet/Optical/InfraRed (UVOIR) photometric points, 64 OIR spectra, and three radio images using 17 different telescopes. We find no promising kilonovae (radioactivity-powered counterparts), and we show how to convert the upper limits to constrain the underlying kilonova luminosity function. Initially, we assume that all GW triggers are bona fide astrophysical events regardless of false-alarm rate and that kilonovae accompanying BNS and NSBH mergers are drawn from a common population; later, we relax these assumptions. Assuming that all kilonovae are at least as luminous as the discovery magnitude of GW170817 (−16.1 mag), we calculate that our joint probability of detecting zero kilonovae is only 4.2%. If we assume that all kilonovae are brighter than −16.6 mag (the extrapolated peak magnitude of GW170817) and fade at a rate of 1 mag day−1 (similar to GW170817), the joint probability of zero detections is 7%. If we separate the NSBH and BNS populations based on the online classifications, the joint probability of zero detections, assuming all kilonovae are brighter than −16.6 mag, is 9.7% for NSBH and 7.9% for BNS mergers. Moreover, no more than <57% (<89%) of putative kilonovae could be brighter than −16.6 mag assuming flat evolution (fading by 1 mag day−1) at the 90% confidence level. If we further take into account the online terrestrial probability for each GW trigger, we find that no more than <68% of putative kilonovae could be brighter than −16.6 mag. Comparing to model grids, we find that some kilonovae must have Mej < 0.03 M⊙, Xlan > 10−4, or ϕ > 30° to be consistent with our limits. We look forward to searches in the fourth GW observing run; even 17 neutron star mergers with only 50% coverage to a depth of −16 mag would constrain the maximum fraction of bright kilonovae to <25%. [ABSTRACT FROM AUTHOR]

The first binary neutron star merger, GW170817, was accompanied by a radioactivity-powered optical/infrared transient called a kilonova. To date, no compelling kilonova has been found in all-sky optical surveys, independently of short gamma-ray burst and gravitational-wave triggers. In this work, we searched the first 23 months of the Zwicky Transient Facility (ZTF) data stream for candidate kilonovae in the form of rapidly evolving transients. We combined ZTF alert queries with forced point-spread-function photometry and nightly flux stacking to increase our sensitivity to faint and fast transients. Automatic queries yielded >11,200 candidates, 24 of which passed quality checks and selection criteria based on a grid of kilonova models tailored for both binary neutron star and neutron star–black hole mergers. None of the candidates in our sample was deemed a possible kilonova after thorough vetting. The sources that passed our selection criteria are dominated by Galactic cataclysmic variables. We identified two fast transients at high Galactic latitude, one of which is the confirmed afterglow of long-duration GRB 190106A, the other is a possible cosmological afterglow. Using a survey simulation code, we constrained the kilonova rate for a range of models including top-hat, linearly decaying light curves, and synthetic light curves obtained with radiative transfer simulations. For prototypical GW170817-like kilonovae, we constrain the rate to be Gpc−3 yr−1 (95% confidence). By assuming a population of kilonovae with the same geometry and composition of GW170817 observed under a uniform viewing angle distribution, we obtained a constraint on the rate of R < 4029 Gpc−3 yr−1. [ABSTRACT FROM AUTHOR]

High-cadence transient surveys are able to capture supernovae closer to their first light than ever before. Applying analytical models to such early emission, we can constrain the progenitor stars' properties. In this paper, we present observations of SN 2018fif (ZTF 18abokyfk). The supernova was discovered close to first light and monitored by the Zwicky Transient Facility (ZTF) and the Neil Gehrels Swift Observatory. Early spectroscopic observations suggest that the progenitor of SN 2018fif was surrounded by relatively small amounts of circumstellar material compared to all previous cases. This particularity, coupled with the high-cadence multiple-band coverage, makes it a good candidate to investigate using shock-cooling models. We employ the SOPRANOS code, an implementation of the model by Sapir & Waxman and its extension to early times by Morag et al. Compared with previous implementations, SOPRANOS has the advantage of including a careful account of the limited temporal validity domain of the shock-cooling model as well as allowing usage of the entirety of the early UV data. We find that the progenitor of SN 2018fif was a large red supergiant with a radius of and an ejected mass of. Our model also gives information on the explosion epoch, the progenitor's inner structure, the shock velocity, and the extinction. The distribution of radii is double-peaked, with smaller radii corresponding to lower values of the extinction, earlier recombination times, and a better match to the early UV data. If these correlations persist in future objects, denser spectroscopic monitoring constraining the time of recombination, as well as accurate UV observations (e.g., with ULTRASAT), will help break the extinction/radius degeneracy and independently determine both. [ABSTRACT FROM AUTHOR]

We introduce a new capability of the Neil Gehrels Swift Observatory to provide event-level data from the Burst Alert Telescope (BAT) on demand in response to transients detected by other instruments. We show that the availability of these data can effectively increase the rate of detections and arcminute localizations of gamma-ray bursts (GRB) like GRB 170817 by >400%. We describe an autonomous spacecraft-commanding pipeline purpose built to enable this science; to our knowledge, this is the first fully autonomous extremely low-latency commanding of a space telescope for scientific purposes. This pipeline has been successfully run in its complete form since 2020 January, and has resulted in the recovery of BAT event data for >800 externally triggered events to date (gravitational waves, GWs; neutrinos; GRBs triggered by other facilities; fast radio bursts; and very high-energy detections), now running with a success rate of ∼90%. We exemplify the utility of this new capability by using the resultant data to (1) set the most sensitive upper limits on prompt 1 s duration short GRB-like emission within ±15 s around the unmodeled GW burst candidate S200114f, and (2) provide an arcminute localization for short GRB 200325A and other bursts. We also show that using data from GUANO to localize GRBs discovered by other instruments, we can increase the net rate of arcminute-localized GRBs by 10%–20% per year. Along with the scientific yield of more sensitive searches for subthreshold GRBs, the new capabilities designed for this project will serve as the foundation for further automation and rapid target of opportunity capabilities for the Swift mission, and have implications for the design of future rapid-response space telescopes. [ABSTRACT FROM AUTHOR]

We present a survey of the early evolution of 12 Type IIn supernovae (SNe IIn) at ultraviolet and visible light wavelengths. We use this survey to constrain the geometry of the circumstellar material (CSM) surrounding SN IIn explosions, which may shed light on their progenitor diversity. In order to distinguish between aspherical and spherical CSM, we estimate the blackbody radius temporal evolution of the SNe IIn of our sample, following the method introduced by Soumagnac et al. We find that higher-luminosity objects tend to show evidence for aspherical CSM. Depending on whether this correlation is due to physical reasons or to some selection bias, we derive a lower limit between 35% and 66% for the fraction of SNe IIn showing evidence for aspherical CSM. This result suggests that asphericity of the CSM surrounding SNe IIn is common—consistent with data from resolved images of stars undergoing considerable mass loss. It should be taken into account for more realistic modeling of these events. [ABSTRACT FROM AUTHOR]

We present 637 low-redshift optical spectra collected by the Berkeley Supernova Ia Program (BSNIP) between 2009 and 2018, almost entirely with the Kast double spectrograph on the Shane 3 m telescope at Lick Observatory. We describe our automated spectral classification scheme and arrive at a final set of 626 spectra (of 242 objects) that are unambiguously classified as belonging to Type Ia supernovae (SNe Ia). Of these, 70 spectra of 30 objects are classified as spectroscopically peculiar (i.e. not matching the spectral signatures of 'normal' SNe Ia) and 79 SNe Ia (covered by 328 spectra) have complementary photometric coverage. The median SN in our final set has one epoch of spectroscopy, has a redshift of 0.0208 (with a low of 0.0007 and high of 0.1921), and is first observed spectroscopically 1.1 d after maximum light. The constituent spectra are of high quality, with a median signal-to-noise ratio of 31.8 pixel−1, and have broad wavelength coverage, with |$\sim\! 95{{\ \rm per\ cent}}$| covering at least 3700–9800 Å. We analyse our data set, focusing on quantitative measurements (e.g. velocities, pseudo-equivalent widths) of the evolution of prominent spectral features in the available early-time and late-time spectra. The data are available to the community, and we encourage future studies to incorporate our spectra in their analyses. [ABSTRACT FROM AUTHOR]

SN 2008es is one of the rare cases of a Type II superluminous supernova (SLSN), showing no narrow features in its early-time spectra, and therefore, its powering mechanism is under debate between circumstellar interaction (CSI) and magnetar spin-down. Late-time data are required for better constraints. We present optical and near-infrared (NIR) photometry obtained from Gemini, Keck, and Palomar Observatories from 192 to 554 d after explosion. Only broad H α emission is detected in a Gemini spectrum at 288 d. The line profile exhibits red-wing attenuation relative to the early-time spectrum. In addition to the cooling SN photosphere, an NIR excess with blackbody temperature ∼1500 K and radius ∼1016 cm is observed. This evidence supports dust condensation in the cool dense shell being responsible for the spectral evolution and NIR excess. We favour CSI, with ∼2–3 M⊙ of circumstellar material (CSM) and ∼10–20 M⊙ of ejecta, as the powering mechanism, which still dominates at our late-time epochs. Both models of uniform density and steady wind fit the data equally well, with an effective CSM radius ∼1015 cm, supporting the efficient conversion of shock energy to radiation by CSI. A low amount (≲0.4 M⊙) of 56Ni is possible but cannot be verified yet, since the light curve is dominated by CSI. The magnetar spin-down powering mechanism cannot be ruled out, but is less favoured because it overpredicts the late-time fluxes and may be inconsistent with the presence of dust. [ABSTRACT FROM AUTHOR]

The bright transient AT2018cow has been unlike any other known type of transient. Its high brightness, rapid rise and decay, and initially nearly featureless spectrum are unprecedented and difficult to explain using models for similar burst sources. We present evidence for faint γ-ray emission continuing for at least 8 d, and featureless spectra in the ultraviolet bands – both unusual for eruptive sources. The X-ray variability of the source has a burst-like character. The UV-optical spectrum does not show any CNO line but is well described by a blackbody. We demonstrate that a model invoking the tidal disruption of a |$0.1\!-\!0.4\, \mathrm{M}_\odot$| helium white dwarf (WD) by a |$10^5\!-\!10^6\, \mathrm{M}_\odot$| black hole located in the outskirts of galaxy Z 137-068 could provide an explanation for most of the characteristics shown in the multiwavelength observations. A blackbody-like emission is emitted from an opaque photosphere, formed by the debris of the WD disruption. Broad features showing up in the optical/infrared spectra in the early stage are probably velocity broadened lines produced in a transient high-velocity outward moving cocoon. The asymmetric optical/infrared lines that appeared at a later stage are emission from an atmospheric layer when it detached from thermal equilibrium with the photosphere, which undergoes more rapid cooling. The photosphere shrinks when its temperature drops, and the subsequent infall of the atmosphere produced asymmetric line profiles. Additionally, a non-thermal jet might be present, emitting X-rays in the 10–150 keV band. [ABSTRACT FROM AUTHOR]

Wide-field optical surveys have begun to uncover large samples of fast (t rise ≲ 5 d), luminous (M peak < −18), blue transients. While commonly attributed to the breakout of a supernova shock into a dense wind, the great distances to the transients of this class found so far have hampered detailed investigation of their properties. We present photometry and spectroscopy from a comprehensive worldwide campaign to observe AT 2018cow (ATLAS 18qqn), the first fast-luminous optical transient to be found in real time at low redshift. Our first spectra (<2 days after discovery) are entirely featureless. A very broad absorption feature suggestive of near-relativistic velocities develops between 3 and 8 days, then disappears. Broad emission features of H and He develop after >10 days. The spectrum remains extremely hot throughout its evolution, and the photospheric radius contracts with time (receding below R < 1014 cm after 1 month). This behaviour does not match that of any known supernova, although a relativistic jet within a fallback supernova could explain some of the observed features. Alternatively, the transient could originate from the disruption of a star by an intermediate-mass black hole, although this would require long-lasting emission of highly super-Eddington thermal radiation. In either case, AT 2018cow suggests that the population of fast luminous transients represents a new class of astrophysical event. Intensive follow-up of this event in its late phases, and of any future events found at comparable distance, will be essential to better constrain their origins. [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]

We present photometry and spectroscopy of SN 2013fs and SN 2013fr in the first ~100 d postexplosion. Both objects showed transient, relatively narrow Hα emission lines characteristic of SNe IIn, but later resembled normal SNe II-P or SNe II-L, indicative of fleeting interaction with circumstellar material (CSM). SN 2013fs was discovered within 8 h of explosion; one of the earliest SNe discovered thus far. Its light curve exhibits a plateau, with spectra revealing strong CSM interaction at early times. It is a less luminous version of the transitional SN IIn PTF11iqb, further demonstrating a continuum of CSM interaction intensity between SNe II-P and SNe IIn. It requires dense CSM within 6.5 x 1014 cm of the progenitor, from a phase of advanced pre-SN mass loss beginning shortly before explosion. Spectropolarimetry of SN 2013fs shows little continuum polarization (~0.5 per cent, consistent with zero), but noticeable line polarization during the plateau phase. SN 2013fr morphed from an SN IIn at early times to an SN II-L. After the first epoch, its narrow lines probably arose from host-galaxy emission, but the bright, narrow Hα emission at early times may be intrinsic to the SN. As for SN 2013fs, this would point to a short-lived phase of strong CSM interaction if proven to be intrinsic, suggesting a continuum between SNe IIn and SNe II-L. It is a low-velocity SN II-L like SN 2009kr, but more luminous. SN 2013fr also developed an infrared excess at later times, due to warm CSM dust that requires a more sustained phase of strong pre-SN mass loss. [ABSTRACT FROM AUTHOR]

Herein we analyse late-time (post-plateau; 103 < t < 1229 d) optical spectra of low-redshift (z < 0.016), hydrogen-rich Type IIP supernovae (SNe IIP). Our newly constructed sample contains 91 nebular spectra of 38 SNe IIP, which is the largest data set of its kind ever analysed in one study, and many of the objects have complementary photometric data. The strongest and most robust result we find is that the luminosities of all spectral features (except those of helium) tend to be higher in objects with steeper late-time V-band decline rates. A steep late-time V-band slope likely arises from less efficient trapping of γ-rays and positrons, which could be caused by multidimensional effects such as clumping of the ejecta or asphericity of the explosion itself. Furthermore, if γ-rays and positrons can escape more easily, then so can photons via the observed emission lines, leading to more luminous spectral features. It is also shown that SNe IIP with larger progenitor stars have ejecta with a more physically extended oxygen layer that is well-mixed with the hydrogen layer. In addition, we find a subset of objects with evidence for asymmetric 56Ni ejection, likely bipolar in shape. We also compare our observations to theoretical late-time spectral models of SNe IIP from two separate groups and find moderate-to-good agreement with both sets of models. Our SNe IIP spectra are consistent with models of 12-15 Mʘ progenitor stars having relatively low metallicity (Z < 0.01). [ABSTRACT FROM AUTHOR]

SN 2006gy was the most luminous supernova (SN) ever observed at the time of its discovery and the first of the newly defined class of superluminous supernovae (SLSNe). The extraordinary energetics of SN 2006gy and all SLSNe (>1051 erg) require either atypically large explosion energies (e.g. pair-instability explosion) or the efficient conversion of kinetic into radiative energy (e.g. shock interaction). The mass-loss characteristics can therefore offer important clues regarding the progenitor system. For the case of SN 2006gy, both a scattered and thermal light echo from circumstellar material (CSM) have been reported at later epochs (day ~800), ruling out the likelihood of a pair-instability event and leading to constraints on the characteristics of the CSM. Owing to the proximity of the SN to the bright host-galaxy nucleus, continued monitoring of the light echo has not been trivial, requiring the high resolution offered by the Hubble Space Telescope (HST) or ground-based adaptive optics (AO). Here, we report detections of SN 2006gy using HST and Keck AO at ~3000 d post-explosion and consider the emission mechanism for the very late-time light curve. While the optical light curve and optical spectral energy distribution are consistent with a continued scattered-light echo, a thermal echo is insufficient to power the K'-band emission by day 3000. Instead, we present evidence for late-time infrared emission from dust that is radiatively heated by CSM interaction within an extremely dense dust shell, and we consider the implications on the CSM characteristics and progenitor system. [ABSTRACT FROM AUTHOR]

We present seven epochs of spectropolarimetry of the Type IIb supernova (SN IIb) 2011dh in M51, spanning 86 d of its evolution. The first epoch was obtained 9 d after the explosion, when the photosphere was still in the depleted hydrogen layer of the stripped-envelope progenitor. Continuum polarization is securely detected at the level of P ≈ 0.5 per cent through day 14 and appears to diminish by day 30, which is different from the prevailing trends suggested by studies of other core-collapse SNe. Time-variable modulations in P and position angle are detected across P-Cygni line features. H α and He i polarization peak after 30 d and exhibit position angles roughly aligned with the earlier continuum, while O i and Ca ii appear to be geometrically distinct. We discuss several possibilities to explain the evolution of the continuum and line polarization, including the potential effects of a tidally deformed progenitor star, aspherical radioactive heating by fast-rising plumes of 56Ni from the core, oblique shock breakout, or scattering by circumstellar material. While these possibilities are plausible and guided by theoretical expectations, they are not unique solutions to the data. The construction of more detailed hydrodynamic and radiative-transfer models that incorporate complex aspherical geometries will be required to further elucidate the nature of the polarized radiation from SN 2011dh and other SNe IIb. [ABSTRACT FROM AUTHOR]

Tidal forces close to massive black holes can violently disrupt stars that make a close approach. These extreme events are discovered via bright X-ray and optical/ultraviolet flares in galactic centres. Prior studies based on modelling decaying flux trends have been able to estimate broad properties, such as the mass accretion rate. Here we report the detection of flows of hot, ionized gas in high-resolution X-ray spectra of a nearby tidal disruption event, ASASSN-14li in the galaxy PGC 043234. Variability within the absorption-dominated spectra indicates that the gas is relatively close to the black hole. Narrow linewidths indicate that the gas does not stretch over a large range of radii, giving a low volume filling factor. Modest outflow speeds of a few hundred kilometres per second are observed; these are below the escape speed from the radius set by variability. The gas flow is consistent with a rotating wind from the inner, super-Eddington region of a nascent accretion disk, or with a filament of disrupted stellar gas near to the apocentre of an elliptical orbit. Flows of this sort are predicted by fundamental analytical theory and more recent numerical simulations. [ABSTRACT FROM AUTHOR]

Type Ia supernovae are destructive explosions of carbon-oxygen white dwarfs. Although they are used empirically to measure cosmological distances, the nature of their progenitors remains mysterious. One of the leading progenitor models, called the single degenerate channel, hypothesizes that a white dwarf accretes matter from a companion star and the resulting increase in its central pressure and temperature ignites thermonuclear explosion. Here we report observations with the Swift Space Telescope of strong but declining ultraviolet emission from a type Ia supernova within four days of its explosion. This emission is consistent with theoretical expectations of collision between material ejected by the supernova and a companion star, and therefore provides evidence that some type Ia supernovae arise from the single degenerate channel. [ABSTRACT FROM AUTHOR]

Low-cost mass-produced sensors and optics have recently made it feasible to build telescope arrays which observe the entire accessible sky simultaneously. In this article, we discuss the scientific motivation for these telescopes, including exoplanets, stellar variability, and extragalactic transients. To provide a concrete example we detail the goals and expectations for the Evryscope, an under-construction 780 MPix telescope which covers 8660 sq. deg. in each 2-minute exposure; each night, 18,400 sq. deg. will be continuously observed for an average of 6 hr. Despite its small 61 mm aperture, the system's large field of view provides an étendue which is ~10% of LSST. The Evryscope, which places 27 separate individual telescopes into a common mount which tracks the entire accessible sky with only one moving part, will return 1%-precision, many-year-length, high-cadence light curves for every accessible star brighter than ~16th magnitude. The camera readout times are short enough to provide nearcontinuous observing, with a 97% survey time efficiency. The array telescope will be capable of detecting transiting exoplanets around every solar-type star brighter than mV = 12, providing at least few-millimagnitude photometric precision in long-term light curves. It will be capable of searching for transiting giant planets around the brightest and most nearby stars, where the planets are much easier to characterize; it will also search for small planets nearby M-dwarfs, for planetary occultations of white dwarfs, and will perform comprehensive nearby microlensing and eclipse-timing searches for exoplanets inaccessible to other planet-finding methods. The Evryscope will also provide comprehensive monitoring of outbursting young stars, white dwarf activity, and stellar activity of all types, along with finding a large sample of very-long-period M-dwarf eclipsing binaries. When relatively rare transients events occur, such as gamma-ray bursts (GRBs), nearby supernovae, or even gravitational wave detections from the Advanced LIGO/Virgo network, the array will return minute-by-minute light curves without needing pointing toward the event as it occurs. By coadding images, the system will reach V ~ 19 in 1-hr integrations, enabling the monitoring of faint objects. Finally, by recording all data, the Evryscope will be able to provide pre-event imaging at 2-minute cadence for bright transients and variable objects, enabling the first high-cadence searches for optical variability before, during and after all-sky events. [ABSTRACT FROM AUTHOR]

A growing subset of Type Ia supernovae (SNe Ia) shows evidence via narrow emission lines for unexpected interaction with a dense circumstellar medium (SNe IIn/Ia-CSM). The precise nature of the progenitor, however, remains debated owing to spectral ambiguities arising from a strong contribution from the CSM interaction. Late-time spectra offer potential insight if the post-shock cold, dense shell becomes sufficiently thin and/or the ejecta begin to cross the reverse shock. To date, only a few high-quality spectra of this kind exist. Here we report on the late-time optical and infrared spectra of the SNe Ia-CSM 2012ca and 2013dn. These SNe Ia-CSM spectra exhibit low [Fe III]/[Fe II] ratios and strong [Ca II] at late epochs. Such characteristics are reminiscent of the super-Chandrasekhar-mass candidate SN 2009dc, for which these features suggested a low-ionization state due to high densities, although the broad Fe features admittedly show similarities to the blue 'quasi-continuum' observed in some core collapse SNe Ibn and IIn. Neither SN 2012ca nor any of the other SNe Ia-CSM in this paper show evidence for broad oxygen, carbon, or magnesium in their spectra. Similar to the interacting Type IIn SN 2005ip, a number of high-ionization lines are identified in SN 2012ca, including [S III], [Ar III], [Ar X], [Fe VIII], [Fe X], and possibly [Fe XI]. The total bolometric energy output does not exceed 1051 erg, but does require a large kinetic-to-radiative conversion efficiency. All of these observations taken together suggest that SNe Ia-CSM are more consistent with a thermonuclear explosion than a core collapse event, although detailed radiative transfer models are certainly necessary to confirm these results. [ABSTRACT FROM AUTHOR]

We present optical photometric and spectroscopic observations of the Type Ibn (SN 2006jc-like) supernova (SN) iPTF13beo. Detected by the intermediate Palomar Transient Factory ∼3 h after the estimated first light, iPTF13beo is the youngest and the most distant (∼430 Mpc) Type Ibn event ever observed. The iPTF13beo light curve is consistent with light curves of other Type Ibn SNe and with light curves of fast Type Ic events, but with a slightly faster rise-time of two days. In addition, the iPTF13beo R-band light curve exhibits a double-peak structure separated by ∼9 d, not observed before in any Type Ibn SN. A low-resolution spectrum taken during the iPTF13beo rising stage is featureless, while a late-time spectrum obtained during the declining stage exhibits narrow and intermediate-width He i and Si ii features with full width at half-maximum ≈2000–5000 km s−1 and is remarkably similar to the prototypical SN Ibn 2006jc spectrum. We suggest that our observations support a model of a massive star exploding in a dense He-rich circumstellar medium (CSM). A shock breakout in a CSM model requires an eruption releasing a total mass of ∼0.1 M⊙ over a time-scale of couple of weeks prior to the SN explosion. [ABSTRACT FROM AUTHOR]

The intermediate Palomar Transient Factory (iPTF) detection of the most recent outburst of the recurrent nova (RN) system RX J0045.4+4154 in the Andromeda galaxy has enabled the unprecedented study of a massive (M > 1.3 M☼) accreting white dwarf (WD). We detected this nova as part of the near-daily iPTF monitoring of M31 to a depth of R ≈ 21 mag and triggered optical photometry, spectroscopy and soft X-ray monitoring of the outburst. Peaking at an absolute magnitude of MR = –6.6 mag, and with a decay time of 1 mag per day, it is a faint and very fast nova. It shows optical emission lines of He/N and expansion velocities of 1900-2600 km s–1 1-4 days after the optical peak. The Swift monitoring of the X-ray evolution revealed a supersoft source (SSS) with kTeff ≈ 90-110 eV that appeared within 5 days after the optical peak, and lasted only 12 days. Most remarkably, this is not the first event from this system, rather it is an RN with a time between outbursts of approximately 1 yr, the shortest known. Recurrent X-ray emission from this binary was detected by ROSAT in 1992 and 1993, and the source was well characterized as a M > 1.3 M☼ WD SSS. Based on the observed recurrence time between different outbursts, the duration and effective temperature of the SS phase, MESA models of accreting WDs allow us to constrain the accretion rate to and WD mass >1.30 M☼. If the WD keeps 30% of the accreted material, it will take less than a Myr to reach core densities high enough for carbon ignition (if made of C/O) or electron capture (if made of O/Ne) to end the binary evolution. [ABSTRACT FROM AUTHOR]

Some supernovae (SNe) may be powered by the interaction of the SN ejecta with a large amount of circumstellar matter (CSM). However, quantitative estimates of the CSM mass around such SNe are missing when the CSM material is optically thick. Specifically, current estimators are sensitive to uncertainties regarding the CSM density profile and the ejecta velocity. Here we outline a method to measure the mass of the optically thick CSM around such SNe. We present new visible-light and X-ray observations of SN 2010jl (PTF 10aaxf), including the first detection of an SN in the hard X-ray band using NuSTAR. The total radiated luminosity of SN 2010jl is extreme—at least 9 × 1050 erg. By modeling the visible-light data, we robustly show that the mass of the circumstellar material within ∼1016 cm of the progenitor of SN 2010jl was in excess of 10 M☼. This mass was likely ejected tens of years prior to the SN explosion. Our modeling suggests that the shock velocity during shock breakout was ∼6000 km s–1, decelerating to ∼2600 km s–1 about 2 yr after maximum light. Furthermore, our late-time NuSTAR and XMM spectra of the SN presumably provide the first direct measurement of SN shock velocity 2 yr after the SN maximum light—measured to be in the range of 2000-4500 km s–1 if the ions and electrons are in equilibrium, and ≳ 2000 km s–1 if they are not in equilibrium. This measurement is in agreement with the shock velocity predicted by our modeling of the visible-light data. Our observations also show that the average radial density distribution of the CSM roughly follows an r–2 law. A possible explanation for the ≳ 10 M☼ of CSM and the wind-like profile is that they are the result of multiple pulsational pair instability events prior to the SN explosion, separated from each other by years. [ABSTRACT FROM AUTHOR]

Only a few cases of Type Ic supernovae (SNe) with high-velocity ejecta (⩾0.2 c) have been discovered and studied. Here, we present our analysis of radio and X-ray observations of the Type Ic SN PTF 12gzk. The radio emission declined less than 10 days after explosion, suggesting SN ejecta expanding at high velocity (∼0.3 c). The radio data also indicate that the density of the circumstellar material (CSM) around the supernova is lower by a factor of ∼10 than the CSM around normal Type Ic SNe. PTF 12gzk may therefore be an intermediate event between a “normal” SN Ic and a gamma-ray-burst-SN-like event. Our observations of this rapidly declining radio SN at a distance of 58 Mpc demonstrates the potential to detect many additional radio SNe, given the new capabilities of the Very Large Array (improved sensitivity and dynamic scheduling), which are currently missed, leading to a biased view of radio SNe Ic. Early optical discovery followed by rapid radio observations would provide a full description of the ejecta velocity distribution and CSM densities around stripped massive star explosions as well as strong clues about the nature of their progenitor stars. [ABSTRACT FROM AUTHOR]

PTF11kx was a Type Ia supernova (SN Ia) that showed time-variable absorption features, including saturated Ca II H and K lines that weakened and eventually went into emission. The strength of the emission component of Ha gradually increased, implying that the SN was undergoing significant interaction with its circumstellar medium (CSM). These features, and many others, were blueshifted slightly and showed a P-Cygni profile, likely indicating that the CSM was directly related to, and probably previously ejected by, the progenitor system itself. These and other observations led Dilday et al. to conclude that PTF11kx came from a symbiotic nova progenitor like RS Oph. In this work we extend the spectral coverage of PTF11kx to 124-680 rest-frame days past maximum brightness. The late-time spectra of PTF11kx are dominated by Ha emission (with widths of full width at half-maximum intensity ≈2000 km s-1), strong Ca ii emission features (~10,000 km s-1 wide), and a blue "quasi-continuum" due to many overlapping narrow lines of Fe ii. Emission from oxygen, He I, and Balmer lines higher than Ha is weak or completely absent at all epochs, leading to large observed Hα/Hβ intensity ratios. The Ha emission appears to increase in strength with time for ~1 yr, but it subsequently decreases significantly along with the Ca ii emission. Our latest spectrum also indicates the possibility of newly formed dust in the system as evidenced by a slight decrease in the red wing of Ha. During the same epochs, multiple narrow emission features from the CSM temporally vary in strength. The weakening of the Ha and Ca II emission at late times is possible evidence that the SN ejecta have overtaken the majority of the CSM and agrees with models of other strongly interacting SNe Ia. The varying narrow emission features, on the other hand, may indicate that the CSM is clumpy or consists of multiple thin shells. [ABSTRACT FROM AUTHOR]

We present photometry and spectroscopy of the Type IIn supernova (SN) 2011ht, identified previously as a possible SN impostor. The light curve exhibits an abrupt transition from a well-defined ∼120 d plateau to a steep bolometric decline, plummeting 4–5 mag in the optical and 2–3 mag in the infrared in only ∼10 d. Leading up to peak brightness (MV = −17.4 mag), a hot emission-line spectrum exhibits strong signs of interaction with circumstellar material (CSM), in the form of relatively narrow P-Cygni features of H i and He i superimposed on broad Lorentzian wings. For the latter half of the plateau phase, the spectrum exhibits strengthening P-Cygni profiles of Fe ii, Ca ii and Hα. By day 147, after the plateau has ended, the SN entered the nebular phase, heralded by the appearance of forbidden transitions of [O i], [O ii] and [Ca ii] over a weak continuum. At this stage, the light curve exhibits a low optical luminosity that is comparable to that of the most subluminous Type II-P supernovae, and a relatively fast visual wavelength decline that appeared to be significantly steeper than the 56Co decay rate. However, the total pseudo-bolometric decline, including the infrared luminosity, is consistent with 56Co decay, and implies a low 56Ni mass in the range 0.006–0.01 M⊙, near the lower end of the range exhibited by SNe II-P. We therefore characterize SN 2011ht as a core-collapse SN very similar to the peculiar SNe IIn 1994W and 2009kn. These three SNe appear to define a subclass, which are Type IIn based on their spectrum, but that also exhibit well-defined plateaus and produce low 56Ni yields. We therefore suggest Type IIn-P as a name for this subclass. The absence of observational signatures of high-velocity material from SNe IIn-P could be the result of an opaque shell at the shocked SN-CSM interface, which remains optically thick longer than the time-scale for the inner ejecta to cool and become transparent. Possible progenitors of SNe IIn-P, consistent with the available data, include 8–10 M⊙ stars, which undergo core collapse as a result of electron capture after a brief phase of enhanced mass loss, or more massive (M ≳ 25 M⊙) progenitors, which experience substantial fallback of the metal-rich radioactive ejecta. In either case, the energy radiated by these three SNe during their plateau (2–3 × 1049 erg for SN 2011ht) must be dominated by CSM interaction, and the subluminous tail is the result of low 56Ni yield. [ABSTRACT FROM AUTHOR]

Some reports of supernova (SN) discoveries turn out not to be true core-collapse explosions. One such case was SN 2009ip, which was recognized to be the eruption of a luminous blue variable (LBV) star. This source had a massive (50–80 M⊙), hot progenitor star identified in pre-explosion data; it had documented evidence of pre-outburst variability and it was subsequently discovered to have a second outburst in 2010. In 2012, the source entered its third known outburst. Initial spectra showed the same narrow-line profiles as before, suggesting another LBV-like eruption. We present new photometry and spectroscopy of SN 2009ip, indicating that the 2012 outburst transitioned into a genuine SN explosion. The most striking aspect of these data is that unlike any previous episodes, the spectrum developed Balmer lines with very broad P-Cygni profiles characteristic of normal Type II supernovae (SNe II), in addition to overlying narrow emission components. The emission lines exhibit unprecedented (for any known non-terminal LBV-like eruption) full width at half-maximum intensity values of ∼8000 km s−1, while the absorption components seen just before the main brightening had blue wings extending out to −13 000 km s−1. These velocities are typical of core-collapse SN explosions, but have never been associated with emission lines from a non-terminal LBV-like eruption. SN 2009ip is the first object to have both a known massive blue progenitor star and LBV-like eruptions with accompanying spectra observed a few years prior to becoming a SN. Immediately after the broad lines first appeared, the peak absolute magnitude of MV ≈ −14.5 was fainter than that of normal SNe II. However, after a brief period of fading, the source quickly brightened again to MR = −17.5 mag in ∼2 d, suggesting a causal link to the prior emergence of the broad-line spectrum. Once the bright phase began, the broad lines mostly disappeared, and the spectrum resembled the early optically thick phases of luminous SNe IIn. The source reached a peak brightness of −18 mag about 2 weeks later, after which broad emission lines again developed in the spectrum as the source faded. We conclude that the most recent 2012 outburst of SN 2009ip was the result of a true core-collapse SN IIn that occurred when the progenitor star was in an LBV-like outburst phase, and where the SN was initially faint and then rapidly brightened due to interaction with circumstellar material. The pulsational pair instability, LBV-like eruptions or other instabilities due to late nuclear burning phases in massive stars may have caused the multiple pre-SN eruptions. [ABSTRACT FROM AUTHOR]

ABSTRACT We present optical photometry and spectroscopy of the peculiar Type IIn/Ibn supernova (SN) 2011hw. Its optical light curve exhibits a slower decline rate than that of normal SNe Ibc, with a peak absolute magnitude of −19.5 (unfiltered) and a secondary rise 20-30 d later of −18.3 mag ( R). Spectra of SN 2011hw are highly unusual compared to those of normal SN types, most closely resembling the spectra of SNe Ibn. We centre our analysis on comparing SN 2011hw to the well-studied Type Ibn SN 2006jc. While the two SNe have many important similarities, the differences are quite telling: compared to SN 2006jc, SN 2011hw has weaker He i and Ca ii lines and relatively stronger H lines, its light curve exhibits a higher visual-wavelength luminosity and slower decline rate, and emission lines associated with the progenitor's circumstellar material (CSM) are narrower. One can reproduce the unusual continuum shape of SN 2011hw with roughly equal contributions from a 6000-K blackbody and a spectrum of SN 2006jc. We attribute this blackbody-like emission component and many other differences between the two SNe to a small amount of additional H in SN 2011hw, analogous to the small H mass that makes SNe IIb differ from SNe Ib. Slower speeds in the CSM and somewhat elevated H content suggest a connection between SN 2011hw's progenitor and Ofpe/WN9 stars, which have been associated with luminous blue variables (LBVs) in their hot quiescent phases, and are H poor - but not H free like classical Wolf-Rayet (WR) stars. Comparisons between SN 2011hw and SN 2006jc can be largely understood if their progenitors exploded at different points in the transitional evolution from an LBV to a WR star. [ABSTRACT FROM AUTHOR]