Alignment Timescale of the Microquasar GRO J1655-40

TL;DR

This paper models the alignment timescale of the black hole's spin in the microquasar GRO J1655-40, considering warped accretion disks and stellar evolution to understand the current misalignment.

Contribution

It provides an analytical model for the alignment timescale incorporating stellar evolution and accretion dynamics, offering insights into the system's current spin-orbit configuration.

Findings

Alignment timescale is comparable to the system's lifetime.

The black hole is most likely close to alignment with the orbital plane.

Current misalignment is consistent with either initial counter-alignment or near-alignment.

Abstract

The microquasar GRO J1655-40 has a black hole with spin angular momentum apparently misaligned to the orbital plane of its companion star. We analytically model the system with a steady state disc warped by Lense-Thirring precession and find the timescale for the alignment of the black hole with the binary orbit. We make detailed stellar evolution models so as to estimate the accretion rate and the lifetime of the system in this state. The secondary can be evolving at the end of the main sequence or across the Hertzsprung gap. The mass-transfer rate is typically fifty times higher in the latter case but we find that, in both cases, the lifetime of the mass transfer state is at most a few times the alignment timescale. The fact that the black hole has not yet aligned with the orbital plane is therefore consistent with either model. We conclude that the system may or may not have been counter-aligned after its supernova kick but that it is most likely to be close to alignment rather than counteralignment now.