Neutron Star–Black Hole Mergers from Gravitational-wave Captures

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, or binary neutron stars. Here we examine single-single (sin-sin) gravitational wave (GW) captures in different types of star clusters-galactic nuclei, globular clusters, and young stellar clusters-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.

Observation of Gravitational Waves from Two Neutron Star–Black Hole Coalescences

“…Models of star formation in the dynamical environments of young star clusters predict NSBH merger rate densities of 0.1-100 Gpc −3 yr −1 (Fragione & Banerjee 2020;Hoang et al 2020;Rastello et al 2020;Santoliquido et al 2020). In this scenario, most systems that form merging NSBHs (∼80%) are ejected without undergoing dynamical exchanges, proceeding to merge in the field.…”

“…Major uncertainties regarding massive binary evolution, such as mass loss, mass transfer, and the impact of supernova explosions result in a wide range of merger rate predictions: 0.1-800 Gpc −3 yr −1 (Belczynski et al 2002(Belczynski et al , 2006(Belczynski et al , 2016Sipior & Sigurdsson 2002;Dominik et al 2015;Eldridge et al 2017;Kruckow et al 2018;Mapelli & Giacobbo 2018;Neijssel et al 2019;Drozda et al 2020;Zevin et al 2020;Broekgaarden et al 2021). Comparable NSBH merger rates are predicted from young star clusters (Ziosi et al 2014;Rastello et al 2020), while NSBH merger rates in globular and nuclear clusters are predicted to be orders of magnitude lower (Clausen et al 2013;Arca Sedda 2020;Fragione & Banerjee 2020;Hoang et al 2020;Ye et al 2020). Measuring the NSBH merger rate and properties such as masses and spins is crucial in determining formation channels.…”

Rates of compact object coalescences

“…Miller and Lauburg (2009) and pointed out that nuclear clusters in the absence of a massive black hole, which are characterised by deeper potentials and hence higher escape velocities than globular clusters, could be promising environments for BH-BH dynamical formation. O' Leary et al (2009), Tsang (2013), Hoang et al (2020), Rasskazov and Kocsis (2019) explored direct gravitational-wave bremsstrahlung captures from 2-body scattering. Other potential contributions include the deposition of stellar-mass black holes near a massive black hole by the infall of a globular cluster (Arca-Sedda and Gualandris 2018).…”

Constraints on compact binary merger evolution from spin-orbit misalignment in gravitational-wave observations

“…In cases where the binary formed via dynamical capture, whether in dense stellar clusters (e.g. Hoang et al 2020;Rastello et al 2020;Ye et al 2020) or AGN discs (e.g. McKernan et al 2020b,a), we can expect an isotropic distribution of tilt angles because the spins of the two constituents are independent of one another and the orbital plane is set by the direction of approach during the capture.…”

“…Combined with the low expected rates of dynamical capture-driven NS-BH mergers (e.g. Arca Sedda 2020; Hoang et al 2020;Ye et al 2020), we consider the isolated binary formation channel as our preferred pathway. In this paper, we investigate the physical processes that may produce significant spin-orbit anti-alignment, and compare the tilt angles of confirmed or candidate NS-BH mergers to the population of BBH mergers presented in (Abbott et al 2021d).…”