Black holes in stellar-mass binary systems: expiating original spin?

TL;DR

This paper examines whether black hole spins align with their accretion discs in binary systems, finding that full alignment is rare in low- and high-mass X-ray binaries but more likely in ultraluminous X-ray binaries, with observational implications.

Contribution

It provides a systematic analysis of spin-disc alignment likelihood in different stellar-mass binary systems, highlighting differences between LMXBs, HMXBs, and ULXs.

Findings

Misalignment persists in most LMXBs and HMXBs.

ULXs tend to approach alignment due to thermal timescale accretion.

Observational absence of eclipses in ULXs supports stellar-mass black hole models.

Abstract

We investigate systematically whether accreting black hole systems are likely to reach global alignment of the black hole spin and its accretion disc with the binary plane. In low-mass X-ray binaries (LMXBs) there is only a modest tendency to reach such global alignment, and it is difficult to achieve fully: except for special initial conditions we expect misalignment of the spin and orbital planes by ~1 radian for most of the LMXB lifetime. The same is expected in high-mass X-ray binaries (HMXBs). A fairly close approach to global alignment is likely in most stellar-mass ultraluminous X-ray binary systems (ULXs) where the companion star fills its Roche lobe and transfers on a thermal timescale to a black hole of lower mass. These systems are unlikely to show orbital eclipses, as their emission cones are close to the hole's spin axis. This offers a potential observational test, as models for ULXs invoking intermediate-mass black holes do predict eclipses for ensembles of > ~10 systems. Recent observational work shows that eclipses are either absent or extremely rare in ULXs, supporting the picture that most ULXs are stellar-mass binaries with companion stars more massive than the accretor.