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.

Over the past five years evidence has mounted that long-duration (> 2 s) γ-ray bursts (GRBs) the most brilliant of all astronomical explosionssignal the collapse of massive stars in our Universe. This evidence was originally based on the probable association of one unusual GRB with a supernova 1 , but now includes the association of GRBs with regions of massive star formation in distant galaxies 2,3 , the appearance of supernova-like 'bumps' in the optical afterglow light curves of several bursts 4-6 and lines of freshly synthesized elements in the spectra of a few X-ray afterglows 7 . These observations support, but do not yet conclusively demonstrate, the idea that long-duration GRBs are associated with the deaths of massive stars, presumably arising from core collapse. Here we report evidence that

We report the discovery and monitoring of the near-infrared counterpart (AT2017gfo) of a binary neutron-star merger event detected as a gravitational wave source by Advanced LIGO/Virgo (GW170817) and as a short gammaray burst by Fermi /GBM and Integral /SPI-ACS (GRB 170817A). The evolution of the transient light is consistent with predictions for the behaviour of a "kilonova/macronova", powered by the radioactive decay of massive neutronrich nuclides created via r-process nucleosynthesis in the neutron-star ejecta. In particular, evidence for this scenario is found from broad features seen in Hubble Space Telescope infrared spectroscopy, similar to those predicted for lanthanide dominated ejecta, and the much slower evolution in the near-infrared K s -band compared to the optical. This indicates that the late-time light is dominated by high-opacity lanthanide-rich ejecta, suggesting nucleosynthesis to the 3rd r-process peak (atomic masses A ≈ 195). This discovery confirms that neutron-star mergers produce kilo-/macronovae and that they are at least a major -if not the dominant -site of rapid neutron capture nucleosynthesis in the universe.

It is thought that the first generations of massive stars in the Universe were an important, and quite possibly dominant 1 , source of the ultra-violet radiation that reionized the hydrogen gas in the intergalactic medium (IGM); a state in which it has remained to the present day. Measurements of cosmic microwave background anisotropies suggest that this phase-change largely took place 2 in the redshift range z=10.8 ±1.4, while observations of quasars and Lyman-α galaxies have shown that the process was essentially completed 3,4,5 by z≈6. However, the detailed history of reionization, and characteristics of the stars and proto-galaxies that drove it, remain unknown. Further progress in understanding requires direct observations of the sources of ultra-violet radiation in the era of reionization, and mapping the evolution of the neutral hydrogen (H I) fraction through time. The detection of galaxies at such redshifts is highly challenging, due to their intrinsic faintness and high luminosity distance, whilst bright quasars appear to be rare It has long been recognised that GRBs have the potential to be powerful probes of the early universe. Known to be the end product of rare massive stars 11 , GRBs and their afterglows can briefly outshine any other source in the universe, and would be theoretically detectable to z ~ 20 and beyond 12,13 . Their association with individual stars means that they serve as a signpost of star formation, even if their host galaxies are too 5 faint to detect directly. Equally important, precise determination of the hydrogen Lyman-α absorption profile can provide a measure of the neutral fraction of the IGM at the location of the burst 9,10,14,15 . With multiple GRBs at z > 7, and hence lines of sight through the IGM, we could thus trace the process of reionization from its early stages.However, until now the highest redshift GRBs (at z = 6. Ground-based optical observations in the r, i and z filters starting within a few minutes of the burst revealed no counterpart at these wavelengths (see Supplementary Information (SI)).The United Kingdom Infrared Telescope (UKIRT) in Hawaii responded to an automated request, and began observations in the K-band 21 minutes post burst. These images ( Figure 1) revealed a point source at the reported X-ray position, which we concluded was likely to be the afterglow of the GRB. We also initiated further nearinfrared (NIR) observations using the Gemini-North 8-m telescope, which started 75 min after the burst, and showed that the counterpart was only visible in filters redder than about 1.2 µm. In this range the afterglow was relatively bright and exhibited a shallow spectral slope F ν ∝ ν -0.26 , in contrast to the deep limit on any flux in the Y filter (0.97-1.07 µm). Later observations from Chile using the MPI/ESO 2.2m telescope, Gemini South and the Very Large Telescope (VLT) confirmed this finding. The nondetection in the Y-band implies a power-law spectral slope between Y and J steeper than. This is impossible for dust at any redshift, and is a tex...

3It is now accepted that long duration γ-ray bursts (GRBs) are produced during the collapse of a massive star 1,2 . 11,12 . GRB 060505 was a faint burst with a duration of 4 s. GRB 060614 had a duration of 102 s and a pronounced hard to soft evolution. Both were rapidly localised by Swift's X-ray telescope (XRT). Subsequent follow-up of these bursts led to the discovery of their optical afterglows, locating them in galaxies at low redshift: GRB 060505 at z = 0.089 13 and GRB 060614 at z = 0.125 14,15 . The relative proximity of these bursts engendered an expectation that a bright SN would be discovered a few days after the bursts, as had been found just a few months before in 4 another low-redshift Swift burst, GRB 060218 (z = 0.033) 9 , and in all previous wellobserved nearby bursts 1,5-8 .We monitored the afterglows of GRB 060505 and 060614 using a range of telescopes (see supplementary material for details). These led to early detections of the afterglows. We continued the monitoring campaign and obtained stringent upper limits on any re-brightening at the position of the optical afterglows up to 12 and 5 weeks after the bursts, respectively. The light-curves obtained based on this monitoring are shown in Fig. 1. For GRB 060505 we detected the optical afterglow at a single epoch. All subsequent observations resulted in deep upper limits. For GRB 060614 we followed the decay of the optical afterglow in the R-band up to four nights after the burst. In later observations no source was detected to deep limits (see also 14,15 for independent studies of this event). As seen in Fig. 1, the upper limits are far below the level seen in previous SNe, in particular previous SNe associated with long-duration GRBs 5-9 . For both GRBs A concern in any attempt to uncover a SN associated with a GRB is the presence of a poorly quantified level of extinction along the line of sight. In these cases however,we are fortunate that the levels of Galactic extinction in both directions are very low,. In the case of GRB 060505, our spatially resolved spectroscopy of the host galaxy allows us to use the Balmer emission line ratios to limit the dust obscuration 5 at the location of the burst. The Balmer line ratio is consistent with no internal reddening. In the case of GRB 060614, the detection of the early afterglow in many bands, including the Swift UV bands UVW1 and UVW2 17 , rules out significant obscuration of the source in the host galaxy and we conclude that there is no significant dust obscuration in either case (see also 15 ).Both GRBs were located in star-forming galaxies. The host galaxy of GRB 060505 has an absolute magnitude of about M B = -19.6 and the spectrum displays the prominent emission lines typically seen in star-forming galaxies. The 2-dimensional spectrum shows that the host galaxy emission seen at the position of the afterglow is due to a compact H II region in a spiral arm of the host (see the supplementary material for details). We estimate a star-formation rate of 1 M  yr −1 and a specific rate of about 4T...