2025ApJ...981...38P

The gravitational-wave (GW) event S230518h is a potential binary neutron star-black hole merger (NSBH) event that was detected during engineering run 15, which served as the commissioning period before the LIGO-Virgo-KAGRA O4a observing run. Despite its low probability of producing detectable electromagnetic emissions, we performed extensive follow-up observations of this event using the Gravitational-wave Electromagnetic Counterpart Korean Observatories (GECKO) telescopes in the Southern Hemisphere. Our observations covered 61.7% of the 90% credible region, a 284 deg² area accessible from the Southern Hemisphere, reaching a median limiting magnitude of R = 21.6 mag. In these images, we conducted a systematic search for an optical counterpart of this event by combining a convolutional-neural-network-based classifier and human verification. We identified 128 transient candidates, but no significant optical counterpart was found that could have caused the GW signal. Furthermore, we provide feasible kilonova properties that are consistent with the upper limits of the observations. Although no optical counterpart was found, our result demonstrates both GECKO's efficient wide-field follow-up capabilities and usefulness for constraining properties of kilonovae from NSBH mergers at distances of ∼200 Mpc.