Program: GN-2021A-Q-102

The discovery of an electromagnetic counterpart to the first binary neutron star merger detected via gravitational waves (GW170817) heralded a new era in multi-messenger astronomy, transforming topics as disparate as nucleosynthesis, cosmology, and tests of general relativity. Although the bulk of our knowledge about host galaxies and relativistic (jetted) emission from compact binary mergers still comes from Swift GRBs with fast and bright X-ray afterglows, the single best-studied merger afterglow is now that of GW170817, which was too faint and too late-rising in X-rays to belong to this observationally biased sample. The most prolific compact binary merger discovery engine, then, is the Fermi Gamma-ray Space Telescope, which detects nearly one short GRB per week. Unfortunately, there is a relative paucity of electromagnetic observations of Fermi GRBs because it is challenging to locate them within their positional uncertainties of tens to thousands of square degrees. Another avenue to discover compact binary mergers is by searching for the quasi-isotropic optical emission of the afterglow and kilonova that follows the merger, which allows us to find r-process nucleosynthesis sites independently of the prompt emission. We are using target of opportunity observations with ZTF to conduct a systematic census of optical counterparts of Fermi short GRBs, while at the same time we are continuously mining the ZTF synoptical survey alert stream to find afterglow- and kilonova-like fast fading transients, likely to be the product of a compact binary merger. Here, we propose to leverage Gemini's unique queue-scheduling capabilities and optical as well as near-infrared sensitivity to cement the association of an optically discovered transient to a compact binary merger and characterize such an event in detail.