1000 Days of GW170817: A Deep Multi-Band View of the First Neutron Star Merger with HST

The recent joint detection of gravitational waves and light from the binary neutron star merger GW170817 [4] has ushered in a new era of discovery. The merger was fi�rst localized by the Advanced Laser Interferometer Gravitational-wave Observatory ([1]; aLIGO) and the Advanced Virgo Observatory ([7]; Virgo). The second messenger came in the form of electromagnetic (EM) emission, spanning the -ray, X-ray, UV, NIR, optical and radio regimes (e.g., [5]). This multi-messenger discovery signi�ed a wealth of �firsts in high-energy astrophysics: the �first direct evidence for the origin of short gamma-ray bursts (sGRBs; [3]), the first evidence for heavy element production in a merger, and the fi�rst evidence for relativistic outflows launched from a merger [25, 33]. The precise localization and redshift from the EM counterpart, coupled with the distance inferred from gravitational waves, also enabled the fi�rst `standard siren' measurement of the Hubble Constant [2]. Finally, observations of GW170817 provided crucial insight into neutron star radii and the equation of state [6]. HST has played a pivotal role in characterizing the optical and near-infrared (NIR) evolution of GW170817, with reported detections spanning � 10-350 days post-merger [8, 17, 21, 22, 28], and a combined total of nearly 50 HST orbits spent on the object. The optical and NIR emission quickly fell below the detection thresholds of the largest ground- based telescopes, due to a combination of observability, intrinsic faintness (� > 26 AB mag) and contaminating light from its bright host galaxy, NGC4993. Thus, all optical/NIR detections and constraining upper limits of the counterpart to GW170817 at � >100 days have been enabled by HST (Fig. 1). Here, we propose 13 orbits of HST/WFC3 imaging to obtain late-time template images in four fi�lters, comprising all fi�lters which have HST imaging beyond � 100 days and for which no adequate template images exist. These images will enable reliable and uniform photometry of the optical/NIR counterpart to GW170817 for the fi�rst time. In conjunction with ongoing radio and X-ray campaigns by the community, this will allow accurate inferred properties of relativistic outflows from the merger, and the deepest limits on a star cluster at the merger position, strongly constraining the binary progenitor's formation.