No Detectable Kilonova Counterpart is Expected for O3 Neutron Star-Black Hole Candidates

TL;DR

This paper assesses the likelihood and brightness of kilonova signals from neutron star-black hole mergers detected via gravitational waves, concluding that detectable counterparts are unlikely with current technology, but other EM signals may be more promising.

Contribution

It provides a detailed analysis of the tidal disruption probabilities and kilonova brightness for several GW-detected NSBH mergers, highlighting the challenges in electromagnetic counterpart detection.

Findings

Most GW-detected NSBH mergers are unlikely to produce detectable kilonovae.

Disrupted events constitute less than 20% of NSBH mergers, mainly due to low black hole spins.

Future observations may better detect gamma-ray bursts and afterglows than kilonovae.

Abstract

We analyse the tidal disruption probability of potential neutron star--black hole (NSBH) merger gravitational wave (GW) events, including GW190426_152155, GW190814, GW200105_162426 and GW200115_042309, detected during the third observing run of the LIGO/Virgo Collaboration, and the detectability of kilonova emission in connection with these events. The posterior distributions of GW190814 and GW200105_162426 show that they must be plunging events and hence no kilonova signal is expected from these events. With the stiffest NS equation of state allowed by the constraint of GW170817 taken into account, the probability that GW190426_152155 and GW200115_042309 can make tidal disruption is $\sim24\%$ and $\sim3\%$, respectively. However, the predicted kilonova brightness is too faint to be detected for present follow-up search campaigns, which explains the lack of electromagnetic (EM) counterpart detection after triggers of these GW events. Based on the best constrained population synthesis simulation results, we find that disrupted events account for only $\lesssim20\%$ of cosmological NSBH mergers since most of the primary BHs could have low spins. The associated kilonovae for those disrupted events are still difficult to be discovered by LSST after GW triggers in the future, because of their low brightness and larger distances. For future GW-triggered multi-messenger observations, potential short-duration gamma-ray bursts and afterglows are more probable EM counterparts of NSBH GW events.