From Localization to Discovery: Bayesian Ranking of Electromagnetic Counterparts to Gravitational-Wave Events

The robust association of electromagnetic candidates discovered during follow-up of gravitational-wave alerts is challenging, not only due to the large sky areas and broad distance uncertainties, but also due to the tens to hundreds of unrelated optical transients that are observed per event. We present a Bayesian ranking method to identify electromagnetic counterparts to GW events using only location information. The framework combines three-dimensional gravitational wave skymaps with host-galaxy information, a morphology-aware host association, empirical offset priors, and peculiar velocity corrections. We apply the method to GW170817 where it ranks AT2017gfo as the top candidate and correctly selects NGC,4993 as the host. The approach is directly applicable to transient candidates with only location information and enables more efficient follow-up with prioritized candidates and leads to more reliable counterpart identification in current and future observing runs.