Searching for Neutron Star Mergers in the Absence of Gravitational Waves with Optical Kilonova and Afterglow Emission

Abstract The LIGO–Virgo–KAGRA gravitational-wave network, which enabled the discovery of the kilonova (KN) counterpart to GW170817 during observing run 2 (O2), has just reached the end of O4 with no new confirmed neutron star mergers, suggesting the intrinsic rate of these events must be even lower than previously estimated. As a result, building a sample of KNe will remain challenging even with continued gravitational wave observations, motivating complementary discovery strategies that do not rely on gravitational-wave triggers. In this work, we consider how leveraging bright short gamma-ray burst afterglows can aid in the discovery of KNe with the Vera C. Rubin Observatory’s upcoming Legacy Survey of Space and Time (LSST), whose unprecedented depth will make such detections feasible. We find that nearly on-axis ( θ view ≤30°) afterglows can enhance KN detection rates in the LSST g band from 2 9 − 21 + 51 yr − 1 to 9 1 − 65 + 160 yr − 1 . We further show how the colors of the observed events can be used to distinguish between neutron star merger counterparts with and without KN emission. This study demonstrates how critical multiwavelength and multisurvey observations are for these rare events, especially without context from gravitational waves. Fortunately, detectable events will likely be discovered near peak with LSST, allowing for rapid follow-up and confirmation. We discuss key uncertainties in our study, particularly the volume rate of merger events, and the degeneracy between the empirically determined explosion energy and ambient medium density.