Population properties and multimessenger prospects of neutron star-black hole mergers following GWTC-3

TL;DR

This paper analyzes the properties of neutron star-black hole mergers detected via gravitational waves, revealing lower black hole masses and spins compared to black hole binaries, and assesses their multimessenger detection prospects and implications for neutron star physics.

Contribution

It provides the first constraints on NSBH population properties using GWTC-3 data and introduces a novel method for multimessenger analysis based on electromagnetic non-detections.

Findings

Black holes in NSBHs are less massive and have smaller spins than in black hole binaries.

Evidence for a mass gap between neutron stars and black holes in NSBH systems.

Fewer than 14% of NSBH mergers detectable in GWs will have electromagnetic counterparts.

Abstract

Neutron star-black hole (NSBH) mergers detected in gravitational waves have the potential to shed light on supernova physics, the dense matter equation of state, and the astrophysical processes that power their potential electromagnetic counterparts. We use the population of four candidate NSBH events detected in gravitational waves so far with a false alarm rate $\leq 1~\mathrm{yr}^{-1}$ to constrain the mass and spin distributions and multimessenger prospects of these systems. We find that the black holes in NSBHs are both less massive and have smaller dimensionless spins than those in black hole binaries. We also find evidence for a mass gap between the most massive neutron stars and least massive black holes in NSBHs at 98.6% credibility. Using an approach driven by gravitational-wave data rather than binary simulations, we find that fewer than 14% of NSBH mergers detectable in gravitational waves will have an electromagnetic counterpart. While the inferred presence of a mass gap and fraction of sources with a counterpart depend on the event selection and prior knowledge of source classification, the conclusion that the black holes in NSBHs have lower masses and smaller spin parameters than those in black hole binaries is robust. Finally, we propose a method for the multimessenger analysis of NSBH mergers based on the nondetection of an electromagnetic counterpart and conclude that, even in the most optimistic case, the constraints on the neutron star equation of state that can be obtained with multimessenger NSBH detections are not competitive with those from gravitational-wave measurements of tides in binary neutron star mergers and radio and X-ray pulsar observations.