Letter to the Editor

Origin of the black hole spin in lower-mass-gap black hole-neutron star binaries

¹ Department of Physics, Anhui Normal University, Wuhu, Anhui 241002, China
² Guangxi Key Laboratory for Relativistic Astrophysics, School of Physical Science and Technology, Guangxi University, Nanning 530004, China
³ Département d’Astronomie, Université de Genève, Chemin Pegasi 51, 1290 Versoix, Switzerland
⁴ Gravitational Wave Science Center (GWSC), Université de Genève, 24 quai E. Ansermet, 1211 Geneva, Switzerland
⁵ Nevada Center for Astrophysics, University of Nevada, Las Vegas, NV 89154, USA
⁶ Department of Physics and Astronomy, University of Nevada, Las Vegas, NV 89154, USA
⁷ School of Physics and Physical Engineering, Qufu Normal University, Qufu, Shandong 273165, China
⁸ Department of Physics, School of Physics and Materials Science, Nanchang University, Nanchang 330031, China
⁹ College of Physics, Guizhou University, Guiyang, Guizhou 550025, PR China

^⋆ Corresponding author; yingqin2013@hotmail.com

Received: 21 September 2024
Accepted: 9 November 2024

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_⊙ and 1.2 − 2.0 M_⊙ 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 that incorporates 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 χ_BH > 0.3. Alternatively, if a nonspinning BH (e.g., M_BH = 3.6 M_⊙) forms first, it can accrete up to ≈0.2 M_⊙ via case BA mass transfer (MT), reaching a spin magnitude of χ_BH ≈ 0.18 under Eddington-limited accretion. With a higher Eddington accretion limit (i.e., 10.0 ̇M_Edd), the BH can attain a significantly higher spin magnitude of χ_BH ≈  0.65 by accreting approximately 1.0 M_⊙ during case BA MT phase.