0 Assigned ID: 559 ----- Attempting Submission 1 (Fri Apr 05 17:25:28 GMT 2019) ----- HST Phase I Proposal 559 (gw170817.apt) successfully submitted. Receipt: # 559-1 ----- Attempting Submission 2 (Fri Apr 05 17:27:02 GMT 2019) ----- 0 ----- Attempting Submission 1 (Thu Jul 25 02:19:26 GMT 2019) ----- 1000 Days of GW170817: A Deep Multi-Band View of the First Neutron Star Merger with HST The joint detection of gravitational waves and light from the binary neutron star merger GW170817 has ushered in a new era of discovery. HST has played a pivotal role in characterizing the optical and near-infrared (NIR) evolution of GW170817, with reported detections of the kilonova and afterglow spanning ~10-350 days post-merger. In particular, at >100 days, the afterglow brightness is extremely faint at >26 mag; thus all detections or constraining upper limits at late times were enabled by HST. One major complication in measuring accurate photometry of the faint optical and NIR afterglow is contaminating light from the host galaxy, which has complex morphological structure. The only way to properly address this issue is by re-visiting the field after the afterglow has faded to obtain deep multi-band template images, against which we can subtract earlier epochs. Motivated by a case study in one of the optical filters, here we propose 13 orbits of HST/WFC3 imaging to obtain late-time template images in four filters, comprising all filters 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 afterglow to GW170817 for the first 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. Given the high-impact nature of GW170817, we waive all proprietary rights associated with data from this proposal. This proposal is for multi-wavelength observations of the optical counterpart of GW170817 with UVIS/F814W (3 orbits), WFC3/F110W (4 orbits), WFC3/F140W (3 orbits), and WFC3/F160W (3 orbits). These observations are for a template observation of the afterglow of GW170817 and the visits may occur at any point during the cycle. These filters and apertures have been purposely chosen to match those of previous observations of this source. We note that GW170817 is a point source on the outskirts of NGC4993, a bright S0 galaxy, and that the expected brightness of GW170817 is >28 mag. PI Fong is an expert in gravitational wave counterparts, short gamma-ray bursts and their host galaxies. Fong has extensive experience with HST data (WFC3, ACS, WFPC2) and is the PI of a former HST program searching for host galaxies of short GRBs. Fong has 2 papers published to date and 2 papers in prep which are solely based on HST data. The data from this program will be treated in a similar manner. Co-I Paterson is a postdoctoral associate working with PI Fong, and is an expert in optical image subtraction, of direct relevance to this program for obtaining accurate photometry from subtracted images. Co-I's Blanchard, Berger and Chornock are both experts in the field of explosive transients and have led their own HST programs. In particular, co-I Blanchard led our team's host galaxy publication for GW170817. Co-I's Margutti and Alexander have led their own studies of GW170817, in particular focusing on radio and X-ray wavelengths. 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