Black hole-neutron star mergers are unlikely multi-messenger sources

TL;DR

This study assesses the likelihood of black hole-neutron star mergers producing observable electromagnetic signals, concluding that such multi-messenger events are improbable given current astrophysical constraints and observational limitations.

Contribution

It provides a statistical analysis of the conditions under which BH-NS mergers can produce EM counterparts, highlighting the unlikelihood of such multi-messenger detections with current data.

Findings

Over 50% of mergers could have EM counterparts if BHs are highly spinning.

High natal BH spins and stiff NS EOS are disfavored by current constraints.

BH-NS mergers are unlikely to be significant multi-messenger sources.

Abstract

The promise by the LIGO/Virgo/Kagra (LVK) collaboration to detect black hole--neutron star (BH--NS) mergers via gravitational wave (GW) emission has recently been fulfilled with the detection of GW200105 and GW200115. Mergers of BH--NS binaries are particularly exciting for their multi-messenger potential, since the GW detection can be followed by an electromagnetic (EM) counterpart (kilonova, gamma-ray burst, afterglow) that can reveal important information on the equation of state (EOS) of NSs and the nature of the BH spin. We carry out a statistical study of the binary stars that evolve to form a BH--NS binary and compute the rate of merger events that can be followed by an EM counterpart. We find that $\gtrsim 50\%$ of the mergers can lead to an EM counterpart only in the case BHs are born highly spinning ($\chi_{\rm BH}\gtrsim 0.7$), while this fraction does not exceed about $30\%$ for stiff NS EOSs and a few percent for soft NS EOSs for low-spinning BHs ($\chi_{\rm BH}\lesssim 0.2$), suggesting that a high rate of EM counterparts of BH--NS would provide support for high natal BH spins. However, the possibilities that BHs are born with near-maximal spins and that NS internal structure is described by a stiff EOS are disfavored by current LVK constraints. Considering that these values only represent an upper limit to observe an EM counterpart due to current observational limitations, as in brightness sensitivity and sky localization, BH--NS mergers are unlikely multi-messenger sources.