Author contributions M.N. wrote the manuscript, led the analysis and obtained the HST and Gemini data. P.K.B. devised the selection algorithm and identified SN2016aps as an interesting source, and analysed the HST images. E.B. advised on the manuscript and leads the overall project. R.C. and K.B. obtained the MDM spectrum and classified SN2016aps. R.M. analysed the UVOT data.

We present the first observations of a Type I superluminous supernova (SLSN) at 1000 days after maximum light. We observed SN 2015bn using the Hubble Space Telescope Advanced Camera for Surveys in the F475W, F625W and F775W filters at 721 days and 1068 days. SN 2015bn is clearly detected and resolved from its compact host, allowing reliable photometry. A galaxy template constructed from these data further enables us to isolate the SLSN flux in deep ground-based imaging. We measure a light curve decline rate at > 700 days of 0.19 ± 0.03 mag (100 d) −1 , much shallower than the earlier evolution, and slower than previous SLSNe (at any phase) or the decay rate of 56 Co. Neither additional radioactive isotopes nor a light echo can consistently account for the slow decline. A spectrum at 1083 days shows the same [O I] λ6300 and [Ca II] λ7300 lines as seen at ∼ 300 − 400 days, with no new features to indicate strong circumstellar interaction. Radio limits with the Very Large Array rule out an extended wind for mass-loss rates 10 −2.7 Ṁ /v 10 10 −1.1 M yr −1 (where v 10 is the wind velocity in units of 10 km s −1 ). The optical light curve is consistent with L ∝ t −4 , which we show is expected for magnetar spin-down with inefficient trapping; furthermore, the evolution matches predictions from earlier magnetar model fits. The opacity to magnetar radiation is constrained at ∼ 0.01 cm 2 g −1 , consistent with photon-matter pair-production over a broad ∼GeV-TeV range. This suggests the magnetar spectral energy distribution, and hence the 'missing energy' leaking from the ejecta, may peak in this range.

Context. Gravitational wave (GW) astronomy has rapidly reached maturity, becoming a fundamental observing window for modern astrophysics. The coalescences of a few tens of black hole (BH) binaries have been detected, while the number of events possibly including a neutron star (NS) is still limited to a few. On 2019 August 14, the LIGO and Virgo interferometers detected a high-significance event labelled S190814bv. A preliminary analysis of the GW data suggests that the event was likely due to the merger of a compact binary system formed by a BH and a NS. Aims. In this paper, we present our extensive search campaign aimed at uncovering the potential optical and near infrared electromagnetic counterpart of S190814bv. We found no convincing electromagnetic counterpart in our data. We therefore use our non-detection to place limits on the properties of the putative outflows that could have been produced by the binary during and after the merger. Methods. Thanks to the three-detector observation of S190814bv, and given the characteristics of the signal, the LIGO and Virgo Collaborations delivered a relatively narrow localisation in low latency – a 50% (90%) credible area of 5 deg2 (23 deg2) – despite the relatively large distance of 267 ± 52 Mpc. ElectromagNetic counterparts of GRAvitational wave sources at the VEry Large Telescope collaboration members carried out an intensive multi-epoch, multi-instrument observational campaign to identify the possible optical and near infrared counterpart of the event. In addition, the ATLAS, GOTO, GRAWITA-VST, Pan-STARRS, and VINROUGE projects also carried out a search on this event. In this paper, we describe the combined observational campaign of these groups. Results. Our observations allow us to place limits on the presence of any counterpart and discuss the implications for the kilonova (KN), which was possibly generated by this NS–BH merger, and for the strategy of future searches. The typical depth of our wide-field observations, which cover most of the projected sky localisation probability (up to 99.8%, depending on the night and filter considered), is r ∼ 22 (resp. K ∼ 21) in the optical (resp. near infrared). We reach deeper limits in a subset of our galaxy-targeted observations, which cover a total ∼50% of the galaxy-mass-weighted localisation probability. Altogether, our observations allow us to exclude a KN with large ejecta mass M ≳ 0.1 M⊙ to a high (> 90%) confidence, and we can exclude much smaller masses in a sub-sample of our observations. This disfavours the tidal disruption of the neutron star during the merger. Conclusions. Despite the sensitive instruments involved in the campaign, given the distance of S190814bv, we could not reach sufficiently deep limits to constrain a KN comparable in luminosity to AT 2017gfo on a large fraction of the localisation probability. This suggests that future (likely common) events at a few hundred megaparsecs will be detected only by large facilities with both a high sensitivity and large field of view. Galaxy-targeted observations can reach the needed depth over a relevant portion of the localisation probability with a smaller investment of resources, but the number of galaxies to be targeted in order to get a fairly complete coverage is large, even in the case of a localisation as good as that of this event.

We present James Webb Space Telescope (JWST) and Hubble Space Telescope (HST) observations of the afterglow of GRB 221009A, the brightest gamma-ray burst (GRB) ever observed. This includes the first mid-IR spectra of any GRB, obtained with JWST/Near Infrared Spectrograph (0.6–5.5 micron) and Mid-Infrared Instrument (5–12 micron), 12 days after the burst. Assuming that the intrinsic spectral slope is a single power law, with F ν ∝ ν −β , we obtain β ≈ 0.35, modified by substantial dust extinction with A V = 4.9. This suggests extinction above the notional Galactic value, possibly due to patchy extinction within the Milky Way or dust in the GRB host galaxy. It further implies that the X-ray and optical/IR regimes are not on the same segment of the synchrotron spectrum of the afterglow. If the cooling break lies between the X-ray and optical/IR, then the temporal decay rates would only match a post-jet-break model, with electron index p < 2, and with the jet expanding into a uniform ISM medium. The shape of the JWST spectrum is near-identical in the optical/near-IR to X-SHOOTER spectroscopy obtained at 0.5 days and to later time observations with HST. The lack of spectral evolution suggests that any accompanying supernova (SN) is either substantially fainter or bluer than SN 1998bw, the proto-type GRB-SN. Our HST observations also reveal a disk-like host galaxy, viewed close to edge-on, that further complicates the isolation of any SN component. The host galaxy appears rather typical among long-GRB hosts and suggests that the extreme properties of GRB 221009A are not directly tied to its galaxy-scale environment.

We present UV/optical photometric and spectroscopic observations of PS16aqv (SN 2016ard), a Type I superluminous supernova (SLSN-I) classified as part of our search for low-z SLSNe. PS16aqv is most similar in timescale and spectroscopic evolution to fast evolving SLSNe-I and reached a peak absolute magnitude of M r ≈ −22.1. The lightcurves exhibit a significant undulation at 30 rest-frame days after peak, with a behavior similar to undulations seen in the slowly fading SLSN-I SN 2015bn. This similarity strengthens the case that fast and slow SLSNe-I form a continuum with a common origin. At ≈ 80 days after peak, the lightcurves exhibit a transition to a slow decline, followed by significant subsequent steepening, indicative of a plateau phase or a second significant undulation. Deep limits at ≈ 280 days after peak imply a tight constraint on the nickel mass, M Ni 0.35 M (lower than for previous SLSNe-I), and indicate that some SLSNe-I do not produce significantly more nickel than normal Type Ic SNe. Using MOSFiT, we model the lightcurve with a magnetar central engine model and find P spin ≈ 0.9 ms, B ≈ 1.5 × 10 14 G, and M ej ≈ 16 M . The implied rapid spin-down time and large reservoir of available energy coupled with the high ejecta mass may account for the fast evolving lightcurve and slow spectroscopic evolution. We also study the location of PS16aqv within its host galaxy and find that it occurred at an offset of 2.46 ± 0.21 kpc from the central, most active star-forming region. We find the host galaxy exhibits low metallicity and spatially varying extinction and star formation rate, with the explosion site of PS16aqv exhibiting lower values than the central region. The complexity seen in the lightcurves of PS16aqv and other events highlights the importance of obtaining well-sampled lightcurves for exploring deviations from a uniform decline.