Origin of Black Hole Spin in Lower-Mass-Gap Black Hole-Neutron Star Binaries

TL;DR

This paper investigates the origins of black hole spin in lower-mass-gap black hole-neutron star binaries using detailed binary evolution models, highlighting how initial conditions and accretion processes influence the black hole's final spin.

Contribution

It introduces a comprehensive binary evolution model with new dynamical tide implementations to explore how black hole spins develop in lower-mass-gap BHNS systems, considering different formation scenarios.

Findings

Rapidly spinning black holes require short initial orbital periods (<0.35 days) if the neutron star forms first.

Non-spinning black holes can reach moderate spins (~0.18) through accretion during Case BA mass transfer.

Higher Eddington accretion limits can produce black holes with spins up to ~0.65 by accreting about 1.0 solar mass.

Abstract

During the fourth observing run, the LIGO-Virgo-KAGRA Collaboration reported the detection of a coalescing compact binary (GW230529$_{-}$181500) with component masses estimated at $2.5-4.5\, M_\odot$ and $1.2-2.0\, M_\odot$ with 90\% credibility. Given the current constraints on the maximum neutron star (NS) mass, this event is most likely a lower-mass-gap (LMG) black hole-neutron star (BHNS) binary. The spin magnitude of the BH, especially when aligned with the orbital angular momentum, is critical in determining whether the NS is tidally disrupted. An LMG BHNS merger with a rapidly spinning BH is an ideal candidate for producing electromagnetic counterparts. However, no such signals have been detected. In this study, we employ a detailed binary evolution model, incorporating new dynamical tide implementations, to explore the origin of BH spin in an LMG BHNS binary. If the NS forms first, the BH progenitor (He-rich star) must begin in orbit shorter than 0.35 days to spin up efficiently, potentially achieving a spin magnitude of $\chi_{\rm BH} > 0.3$. Alternatively, if a non-spinning BH (e.g., $M_{\rm BH} = 3.6\, M_\odot$) forms first, it can accrete up to $\sim 0.2\, M_\odot$ via Case BA mass transfer (MT), reaching a spin magnitude of $\chi_{\rm BH} \sim 0.18$ under Eddington-limited accretion. With a higher Eddington accretion limit (i.e., 10.0 $\Dot{M}_{\rm Edd}$), the BH can attain a significantly higher spin magnitude of $\chi_{\rm BH} \sim\,0.65$ by accreting approximately $1.0\, M_\odot$ during Case BA MT phase.