Kilonova emission from GW230529 and mass gap neutron star-black hole mergers

The detection of the gravitational wave event GW230529, presumably a neutron star-black hole (NSBH) merger, by the LIGO-Virgo-KAGRA Collaboration marks an exciting discovery for multimessenger astronomy. The black hole (BH) has a high probability of falling within the ``mass gap'' (mg) between the neutron star and the BH mass distributions. Because of the relatively low primary mass, this system has a higher likelihood of producing an electromagnetic counterpart than previously detected NSBH mergers. We analyze the potential kilonova (KN) emission from GW230529 and find that, if the source was an NSBH merger, there is a $\ensuremath{\sim}2--28%$ probability (depending on the assumed equation of state) that it produced a KN peaking at $\ensuremath{\sim}1\text{ }\text{ }\mathrm{day}$ postmerger with $g\ensuremath{\lesssim}23.5$ and $i<23$. Hence, it could have been detected by ground-based telescopes. If instead the event was a binary neutron star merger, the probability of KN production drops to $\ensuremath{\sim}0--10%$. Motivated by these results, we simulate a broader population of mass gap NSBH mergers expected during the fifth LIGO/Virgo/KAGRA observing run (O5) and find a 2%--3% chance of KN production per event. Such KNe would typically be fainter than GW230529, with $g\ensuremath{\lesssim}26$ and $i\ensuremath{\lesssim}25$. Based on these findings, DECam-like instruments may be able to detect up to $\ensuremath{\sim}70%$ of future mgNSBH KNe, corresponding to 1--2 multimessenger mgNSBH per year in O5.