GECKO Follow-up Observation of the Binary Neutron Star-Black Hole Merger Candidate S230518h

TL;DR

This study reports on extensive optical follow-up observations of the GW event S230518h, a potential neutron star-black hole merger, using the GECKO telescopes, but finds no optical counterpart despite covering a large sky area.

Contribution

First wide-field follow-up of S230518h with the GECKO telescopes demonstrating effective search capabilities and constraining kilonova properties at ~200 Mpc.

Findings

No optical counterpart detected in the observed region.

Coverage of 61.7% of the 90% credible region.

Set upper limits on kilonova brightness.

Abstract

The gravitational wave (GW) event S230518h is a potential binary neutron star-black hole merger (NSBH) event that was detected during engineering run 15 (ER15), which served as the commissioning period before the LIGO-Virgo-KAGRA (LVK) O4a observing run. Despite its low probability of producing detectable electromagnetic emissions, we performed extensive follow-up observations of this event using the GECKO telescopes in the southern hemisphere. Our observation covered 61.7\% of the 90\% credible region, a $\rm 284\:deg^2$ 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 CNN-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 KN properties that are consistent with the upper limits of observation. 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 $\sim 200$ Mpc.