Neutron Star-Black Hole Mergers from Gravitational Wave Captures

TL;DR

This paper investigates neutron star-black hole mergers from gravitational wave captures in different star clusters, finding that this channel likely contributes little to the overall merger rate compared to other formation scenarios.

Contribution

It provides a comparative analysis of single-single gravitational wave capture rates across various star cluster environments, highlighting their limited role in NSBH mergers.

Findings

Single-single GW capture unlikely to dominate NSBH merger rates.

Other channels like isolated binary evolution may be more significant.

Uncertainties prevent definitive conclusions about merger origins.

Abstract

LIGO's third observing run (O3) has reported several neutron star-black hole (NSBH) merger candidates. From a theoretical point of view, NSBH mergers have received less attention in the community than either binary black holes (BBHs), or binary neutron stars (BNSs). Here we examine single-single (sin-sin) gravitational wave (GW) captures in different types of star clusters -- galactic nuclei (GN), globular clusters (GC), and young stellar clusters (YSC) -- and compare the merger rates from this channel to other proposed merger channels in the literature. There are currently large uncertainties associated with every merger channel, making a definitive conclusion about the origin of NSBH mergers impossible. However, keeping these uncertainties in mind, we find that sin-sin GW capture is unlikely to significantly contribute to the overall NSBH merger rate. In general, it appears that isolated binary evolution in the field or in clusters, and dynamically interacting binaries in triple configurations, may result in a higher merger rate.