Population Properties of Gravitational-Wave Neutron Star--Black Hole Mergers

TL;DR

This paper analyzes the population properties of gravitational-wave neutron star--black hole mergers from LIGO-Virgo-KAGRA data, revealing mass and spin distributions consistent with certain astrophysical models and supporting the isolated formation channel.

Contribution

It provides the first detailed population analysis of GW NSBH mergers, characterizing their mass and spin distributions and implications for their formation channels.

Findings

Primary black hole mass distribution fits a power-law with a high-mass Gaussian component.

Neutron star masses are near flat between 1.0-2.1 solar masses.

Effective spins are near zero, indicating most events are plunging mergers.

Abstract

Over the course of the third observing run of LIGO-Virgo-KAGRA Collaboration, several gravitational-wave (GW) neutron star--black hole (NSBH) candidates have been announced. By assuming these candidates are real signals and of astrophysical origins, we analyze the population properties of the mass and spin distributions for GW NSBH mergers. We find that the primary BH mass distribution of NSBH systems, whose shape is consistent with that inferred from the GW binary BH (BBH) primaries, can be well described as a power-law with an index of $\alpha = 4.8^{+4.5}_{-2.8}$ plus a high-mass Gaussian component peaking at $\sim33^{+14}_{-9}\,M_\odot$. The NS mass spectrum could be shaped as a near flat distribution between $\sim1.0-2.1\,M_\odot$. The constrained NS maximum mass agrees with that inferred from NSs in our Galaxy. If GW190814 and GW200210 are NSBH mergers, the posterior results of the NS maximum mass would be always larger than $\sim2.5\,M_\odot$ and significantly deviate from that inferred in the Galactic NSs. The effective inspiral spin and effective precession spin of GW NSBH mergers are measured to potentially have near-zero distributions. The negligible spins for GW NSBH mergers imply that most events in the universe should be plunging events, which supports the standard isolated formation channel of NSBH binaries. More NSBH mergers to be discovered in the fourth observing run would help to more precisely model the population properties of cosmological NSBH mergers.