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Abstract

The detection of the gravitational-wave event GW230529, presumably a neutron
star-black hole (NSBH) merger, by the LIGO-Virgo-KAGRA (LVK) Collaboration is
an exciting discovery for multimessenger astronomy. The black hole (BH) has a
high probability of falling within the ''mass gap'' between the peaks of the
neutron star (NS) and the BH mass distributions. Because of the low primary
mass, the binary is more likely to produce an electromagnetic counterpart than
previously detected NSBH mergers. We investigate the possible kilonova (KN)
emission from GW230529, and find that if it was an NSBH, there is a $\sim$
2-41% probability (depending on the assumed equation of state) that GW230925
produced a KN with magnitude peaking at $\sim 1-2$ day post merger at $g
\lesssim 23.5$, $i<23$. Hence, it could have been detected by ground-based
telescopes. If it was a binary neutron star (BNS) merger, we find $\sim$ 0-12%
probability that it produced a KN. Motivated by these numbers, we simulated a
broader population of mgNSBH mergers that may be detected in O4, and we
obtained a 9-21% chance of producing a KN, which would be detectable with
$g\lesssim 25$ and $ i \lesssim 24$, typically fainter than what is expected
from GW230529. Based on these findings, DECam-like instruments may be able to
detect up to 80% of future mgNSBH KNe, thus up to $\sim1$ multimessenger mgNSBH
per year may be discoverable at the current level of sensitivity (O4).