On 2017 August 17 a binary neutron star coalescence candidate (later designated GW170817) with merger time 12:41:04 UTC was observed through gravitational waves by the Advanced LIGO and Advanced Virgo detectors. The Fermi Gamma-ray Burst Monitor independently detected a gamma-ray burst (GRB 170817A) with a time delay of ∼ 1.7 s with respect to the merger time. From the gravitational-wave signal, the source was initially localized to a sky region of 31 deg2 at a luminosity distance of 40 − 8 + 8 Mpc and with component masses consistent with neutron stars. The component masses were later measured to be in the range 0.86 to 2.26 M ⊙ . An extensive observing campaign was launched across the electromagnetic spectrum leading to the discovery of a bright optical transient (SSS17a, now with the IAU identification of AT 2017gfo) in NGC 4993 (at ∼ 40 Mpc ) less than 11 hours after the merger by the One-Meter, Two Hemisphere (1M2H) team using the 1 m Swope Telescope. The optical transient was independently detected by multiple teams within an hour. Subsequent observations targeted the object and its environment. Early ultraviolet observations revealed a blue transient that faded within 48 hours. Optical and infrared observations showed a redward evolution over ∼10 days. Following early non-detections, X-ray and radio emission were discovered at the transient’s position ∼ 9 and ∼ 16 days, respectively, after the merger. Both the X-ray and radio emission likely arise from a physical process that is distinct from the one that generates the UV/optical/near-infrared emission. No ultra-high-energy gamma-rays and no neutrino candidates consistent with the source were found in follow-up searches. These observations support the hypothesis that GW170817 was produced by the merger of two neutron stars in NGC 4993 followed by a short gamma-ray burst (GRB 170817A) and a kilonova/macronova powered by the radioactive decay of r-process nuclei synthesized in the ejecta.

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to further constraining the astrophysics of the sources. However, the evidence supporting this progenitor hypothesis is essentially circumstantial: principally that many SGRBs seem to reside in host galaxies, or regions within their hosts, lacking ongoing star formation, thus making a massive star origin unlikely (in contrast to long-duration bursts, which arise in the core-collapse of some short-lived massive stars 16 ).Unfortunately, progress in studying SGRBs has been slow; Swift only localises a handful per year, and they are typically faint, with no optical afterglow or unambiguous host galaxy found in some cases despite rapid and deep searches. Another proposed signature of a NS-NS/NS-BH binary merger is the production of a so-called "kilonova" (sometimes also termed a "macronova" or "r-process supernova") due to the decay of radioactive species produced and initially ejected during the merger process -in other words, an event similar to a faint, short-lived supernova 6-8 . Detailed calculations suggest that the spectra of such kilonova sources will be determined by the heavy r-process ions created in the neutron-rich material.Although these models 10-13 are still far from being fully realistic, a robust conclusion is that the optical flux will be greatly diminished by line-blanketing in the rapidly expanding ejecta, with the radiation emerging instead in the nIR, and stretched out over a longer time scale than would otherwise be the case. This makes previous limits on early optical kilonova emission unsurprising 23 . Specifically, the nIR light curves are expected to exhibit a broad peak, rising after a few days and lasting a week or more in the rest frame. The relatively modest redshift and intensive study of SGRB 130603Bmade it a prime candidate for searching for such a kilonova. Page 3 of 16We imaged the location of the burst with the NASA/ESA Hubble Space Telescope (HST) at two epochs, the first ≈9 days post-burst, and the second at ≈30 days.On each occasion, a single orbit integration was obtained in both the optical F606W filter (0.6 µm) and the nIR F160W filter (1.6 µm) (full details of the imaging and photometric analysis discussed here are given in the Supplementary Information). The HST images are shown in Fig. 1; the key result is seen in the difference frames (right hand panels) that provide clear evidence for a compact transient source in the nIR in epoch 1 (we note that this source was also identified as a candidate kilonova in independent analysis of our epoch 1 data 24 ), which has apparently disappeared by epoch 2 and is absent to the depth of the data in the optical. In order to assess the significance of this result it is important to establish whether any emission seen in the first HST epoch could have a contribution from the SGRB afterglow. A compilation of optical and nIR photometry, gathered by a variety of ground-based telescopes in the few days following the burst, is plotted in Fig. 2, along with our HST results. Although initially bright, the optical afterglow light curve declin...