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.

Extended emission (EE) is a high-energy, early time rebrightening sometimes seen in the light curves of short gamma-ray bursts (GRBs). We present the first contiguous fits to the EE tail and the later X-ray plateau, unified within a single model. Our central engine is a magnetar surrounded by a fall-back accretion disc, formed by either the merger of two compact objects or the accretion-induced collapse of a white dwarf. During the EE phase, material is accelerated to super-Keplarian velocities and ejected from the system by the rapidly rotating (P ≈ 1 − 10 ms) and very strong (10 15 G) magnetic field in a process known as magnetic propellering. The X-ray plateau is modelled as magnetic dipole spin-down emission. We first explore the range of GRB phenomena that the propeller could potentially reproduce, using a series of template light curves to devise a classification scheme based on phenomology. We then obtain fits to the light curves of 9 GRBs with EE, simultaneously fitting both the propeller and the magnetic dipole spin-down and finding typical disc masses of a few 10 −3 M ⊙ to a few 10 −2 M ⊙ . This is done for ballistic, viscous disc and exponential accretion rates. We find that the conversion efficiency from kinetic energy to EM emission for propellered material needs to be 10% and that the best fitting results come from an exponential accretion profile.

We present observations of the optical afterglow of GRB 170817A, made by the Hubble Space Telescope, between February and August 2018, up to one year after the neutron star merger, GW170817. The afterglow shows a rapid decline beyond 170 days, and confirms the jet origin for the observed outflow, in contrast to more slowly declining expectations for 'failed-jet' scenarios. We show here that the broadband (radio, optical, X-ray) afterglow is consistent with a structured outflow where an ultrarelativistic jet, with Lorentz factor Γ 100, forms a narrow core (∼ 5 • ) and is surrounded by a wider angular component that extends to ∼ 15 • , which is itself relativistic (Γ 5). For a two-component model of this structure, the late-time optical decline, where F ∝ t −α , is α = 2.20 ± 0.18, and for a Gaussian structure the decline is α = 2.45 ± 0.23. We find the Gaussian model to be consistent with both the early ∼ 10 days and late 290 days data. The agreement of the optical light curve with the evolution of the broadband spectral energy distribution, and its continued decline, indicates that the optical flux is arising primarily from the afterglow and not any underlying host system. This provides the deepest limits on any host stellar cluster, with a luminosity 4000L (M F606W −4.3).

The binary neutron star merger GW170817 was the first multi-messenger event observed in both gravitational and electromagnetic waves. 1,2 The electromagnetic signal began ∼ 2 seconds post-merger with a weak, short burst of gamma-rays, 3 which was followed over the next hours and days by the ultraviolet, optical and near-infrared emission from a radioactivelypowered kilonova. [4][5][6][7][8][9][10][11] Later, non-thermal rising X-ray and radio emission was observed. 12,13 The low luminosity of the gamma-rays and the rising non-thermal flux from the source at late times could indicate that we are outside the opening angle of the beamed relativistic jet. Alternatively, the emission could be arising from a cocoon of material formed from the interaction between a jet and the merger ejecta. [13][14][15] Here we present late-time optical detections and deep near-infrared limits on the emission from GW170817 at 110 days post-merger. Our new observations are at odds with expectations of late-time emission from kilonova models, being too bright and blue. 16,17 Instead, the emission arises from the interaction between the relativistic ejecta of GW170817 and the interstellar medium. We show that this emission matches the expectations of a Gaussian structured relativistic jet, which would have launched a high luminosity short GRB to an aligned observer. However, other jet structure or cocoon models can also match current data -the future evolution of the afterglow will directly distinguish the origin of the emission.For the Hubble Space Telescope (HST), the end of Sun constraint for GW170817 was on 6 December 2017 (∼ 110 rest-frame days post-merger), and we immediately obtained deep observations in the optical and infrared (see Table 1 and Methods for details of the observations and reduction). The new images were astrometrically aligned to our earlier epoch HST data in order to accurately locate the merger site and perform photometry (see Methods). Images of the merger site in each of our filters are shown in Figure 1. We detect emission at the location of the merger in the optical F606W and F814W filters (central wavelengths, λ cen ∼ 589, 802 nm, respectively). For the near-IR filters F140W and F160W (λ cen ∼ 1392, 1527 nm, respectively) we could not establish significant detections and so can place only upper limits on the transient flux at these wavelengths. Optical and near-infrared light curves for the counterpart to GW170817, including our recent observations, are shown in Figure 2.A detection in the optical or near-IR at such late times is not expected from the family of kilonova models currently in use. Indeed, most detailed studies stop at ∼ 30 days where predicted luminosities correspond to 30 mag, 16, 18 undetectable for even HST. Alternative models of kilonova emission with a slower decay of the light curves 17 would nevertheless predict redder emission than we observe. Initially blue, with M r,AB − M H,AB 0.4 mag at 1.5 days 19 , GW170817 evolved to become very red, with M F606W,AB − M F160W,AB = 2.8 mag at 11 d...

Extended emission gamma-ray bursts are a subset of the 'short' class of burst which exhibit an early time rebrightening of gamma emission in their light curves. This extended emission arises just after the initial emission spike, and can persist for up to hundreds of seconds after trigger. When their light curves are overlaid, our sample of 14 extended emission bursts show a remarkable uniformity in their evolution, strongly suggesting a common central engine powering the emission. One potential central engine capable of this is a highly magnetized, rapidly rotating neutron star, known as a magnetar. Magnetars can be formed by two compact objects coallescing, a scenario which is one of the leading progenitor models for short bursts in general. Assuming a magnetar is formed, we gain a value for the magnetic field and late time spin period for 9 of the extended emission bursts by fitting the magnetic dipole spin-down model of Zhang & Mészáros (2001). Assuming the magnetic field is constant, and the observed energy release during extended emission is entirely due to the spin-down of this magnetar, we then derive the spin period at birth for the sample. We find all birth spin periods are in good agreement with those predicted for a newly born magnetar.

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.

We present Hubble Space Telescope (HST) and Chandra imaging, combined with Very Large Telescope MUSE integral field spectroscopy of the counterpart and host galaxy of the first binary neutron star merger detected via gravitational-wave emission by LIGO and Virgo, GW170817. The host galaxy, NGC 4993, is an S0 galaxy at z=0.009783. There is evidence for large, face-on spiral shells in continuum imaging, and edge-on spiral features visible in nebular emission lines. This suggests that NGC 4993 has undergone a relatively recent ( 1  Gyr) "dry" merger. This merger may provide the fuel for a weak active nucleus seen in Chandra imaging. At the location of the counterpart, HST imaging implies there is no globular or young stellar cluster, with a limit of a few thousand solar masses for any young system. The population in the vicinity is predominantly old with 1% of any light arising from a population with ages 500 Myr < . Both the host galaxy properties and those of the transient location are consistent with the distributions seen for short-duration gamma-ray bursts, although the source position lies well within the effective radius (r 3 e~k pc), providing an r e -normalized offset that is closer than 90% of short GRBs. For the long delay time implied by the stellar population, this suggests that the kick velocity was significantly less than the galaxy escape velocity. We do not see any narrow host galaxy interstellar medium features within the counterpart spectrum, implying low extinction, and that the binary may lie in front of the bulk of the host galaxy.

Gamma-ray bursts (GRBs) are divided into two populations [1, 2]; long GRBs that derive from the core-collapse of massive stars [e.g., 3] and short GRBs that form in the merger of two compact objects [4]. While it is common to divide the two populations at a γ-ray duration of two seconds, classification based on duration does not always cleanly map to the progenitor. This is notable in the form of GRBs with bright,