We report our identification of the optical afterglow and host galaxy of the short-duration gammaray burst sGRB 160821B. The spectroscopic redshift of the host is z = 0.162, making it one of the lowest redshift sGRBs identified by Swift. Our intensive follow-up campaign using a range of groundbased facilities as well as HST, XMM-Newton and Swift, shows evidence for a late-time excess of optical and near-infrared emission in addition to a complex afterglow. The afterglow light-curve at X-ray frequencies reveals a narrow jet, θ j ∼ 1.9 +0.10 −0.03 deg, that is refreshed at > 1 day post-burst by a slower outflow with significantly more energy than the initial outflow that produced the main GRB. Observations of the 5 GHz radio afterglow shows a reverse shock into a mildly magnetised shell. The optical and near-infrared excess is fainter than AT2017gfo associated with GW170817, and is well explained by a kilonova with dynamic ejecta mass M dyn = (1.0 ± 0.6) × 10 −3 M and a secular Corresponding author: G. P. Lamb gpl6@leicester.ac.uk arXiv:1905.02159v3 [astro-ph.HE] 5 Aug 2019 2 Lamb et al.(postmerger) ejecta mass with M pm = (1.0 ± 0.6) × 10 −2 M , consistent with a binary neutron star merger resulting in a short-lived massive neutron star. This optical and near-infrared dataset provides the best-sampled kilonova light-curve without a gravitational wave trigger to date.

The origin of thermal optical and UV emission from stellar tidal disruption flares (TDFs) remains an open question. We present Hubble Space Telescope far-UV (FUV) observations of eight optical/UV-selected TDFs 5–10 yr post-peak. Six sources are cleanly detected, showing point-like FUV emission ( ) from the centers of their host galaxies. We discover that the light curves of TDFs from low-mass black holes (<106.5 M ⊙) show significant late-time flattening. Conversely, FUV light curves from high-mass black hole TDFs are generally consistent with an extrapolation from the early-time light curve. The observed late-time emission cannot be explained by existing models for early-time TDF light curves (i.e., reprocessing or circularization shocks), but is instead consistent with a viscously spreading, unobscured accretion disk. These disk models can only reproduce the observed FUV luminosities, however, if they are assumed to be thermally and viscously stable, in contrast to the simplest predictions of α-disk theory. For one TDF in our sample, we measure an upper limit to the UV luminosity that is significantly lower than expectations from theoretical modeling and an extrapolation of the early-time light curve. This dearth of late-time emission could be due to a disk instability/state change absent in the rest of the sample. The disk models that explain the late-time UV detections solve the TDF “missing energy problem” by radiating a rest-mass energy of ∼0.1 M ⊙ over a period of decades, primarily in extreme UV wavelengths.

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We present observations of ZTF18abfcmjw (SN2019dge), a helium-rich supernova with a fast-evolving light curve indicating an extremely low ejecta mass (≈0.33 M e) and low kinetic energy (≈1.3×10 50 erg). Early-time (<4 days after explosion) photometry reveals evidence of shock cooling from an extended helium-rich envelope of∼0.1 M e located ∼1.2×10 13 cm from the progenitor. Early-time He II line emission and subsequent spectra show signatures of interaction with helium-rich circumstellar material, which extends from5×10 13 cm to2×10 16 cm. We interpret SN2019dge as a helium-rich supernova from an ultra-stripped progenitor, which originates from a close binary system consisting of a mass-losing helium star and a low-mass main-sequence star or a compact object (i.e., a white dwarf, a neutron star, or a black hole). We infer that the local volumetric birth rate of 19dge-like ultra-stripped SNe is in the range of 1400-8200-Gpc yr 3 1 (i.e., 2%-12% of core-collapse supernova rate). This can be compared to the observed coalescence rate of compact neutron star binaries that are not formed by dynamical capture.

Authors affiliations can be found at the end of the text.Long duration gamma-ray bursts (GRBs) mark the birth of a compact object, a neutron star or black hole. At low-redshift (z < 0.2) these events are extremely rare and most are poorly known. Four nearby GRBs have been associated with Type Ic supernovae (SNe Ic), [1,2,3,4,5,6,7]. GRB/SNe lack hydrogen and helium, and are classified as SNe Ic, but have extremely broad features, which indicate the presence of material at very high velocities (> 0.3c) [8]. They have a kinetic energy (E k ) of ∼ > 10 52 erg, and are thought to be the explosion of bare carbon-oxygen cores of stars with initial mass 35-50 M [9]. Here we report observations of the nearby GRB 161219B (z = 0.1475, [10]) and the associated SN2016jca. We obtained a high-cadence time-series of spectra and photometry starting 2 days after explosion. The GRB afterglow had a late achromatic break 12 days after outburst which indicates that the relativistic material was ejected in an outflow with a large opening angle. We first identified the SN 5 days after the GRB [11]. Such an early detection gives us the opportunity to explore the outermost layers of the ejecta. We find the outer most ejecta are dominated by heavy elements, while lighter elements are present in high abundance at low velocities. Geometrically this suggests that we are viewing a high velocity nuclearly processed outflow down its axis. This and the wide opening angle suggests a highly magnetized millisecond pulsar may power the explosion.GRB 161219B exploded on 19-12-2016. Our HST image (Figure 1) shows that SN 2016jca is located in an edge-on spiral galaxy; an analysis of the host galaxy emission lines at the location of the transient, based on a VLT X-Shooter spectrum, indicates a metallicity range 0.3 < Z/Z < 0.55. This is characteristic of other low-redshift gamma-ray burst host galaxies [20,21,22,23].Ten spectra of the optical transient were obtained between 1.99 and 32.65 days (rest frame) after explosion. The first spectra are dominated by the afterglow, and at t = 5.52 d the SN is clearly visible. The spectral features are typical of previously observed Supplementary material can be found at https://www.astro.ljmu.ac.uk/ astcasha/SN2016jcasupplemantary.pdf arXiv:1702.04339v2 [astro-ph.HE]