Enabling kilonova science with Nancy Grace Roman Space Telescope

Binary neutron star mergers and neutron star-black hole mergers are multi-messenger sources that can be detected in gravitational waves and in electromagnetic radiation. The low electron fraction of neutron star merger ejecta favors the production of heavy elements such as lanthanides and actinides via rapid neutron capture (r-process). The decay of these unstable nuclei powers an infrared-bright transient called a "kilonova". The discovery of a population of kilonovae will allow us to determine if neutron star mergers are the dominant sites for r-process element nucleosynthesis, constrain the equation of state of nuclear matter, and make independent measurements of the Hubble constant. The Nancy Grace Roman Space Telescope (Roman) will have a unique combination of depth, near-infrared sensitivity, and wide field of view. These characteristics will enable Roman's discovery of GW counterparts that will be missed by optical telescopes, such as kilonova that are associated with large distances, high lanthanide fractions, high binary mass-ratios, large dust extinction in the line of sight, or that are observed from equatorial viewing angles. In preparation for Roman's launch and operations, our analysis suggests to (i) make available a rapid (∼ 1 week) Target of Opportunity mode for GW follow-up; (ii) include observations of the High Latitude Time-Domain survey footprint in at least two filters (preferably the F158 and F213 filters) with a cadence of ≲ 8 days; (iii) operate in synergy with Rubin Observatory. Following these recommendations, we expect that 1-6 kilonovae can be identified by Roman via target of opportunity observations of well localized (A < 10 deg², 90% C.I.) neutron star mergers during 1.5 years of the LIGO-Virgo-KAGRA fifth (or ∼4-21 in during the sixth) observing run. A sample of 5-40 serendipitously discovered kilonovae can be collected in a 5-year high latitude survey.