Effect of geometrically thin discs on precessing, thick flows: Relevance to type-C QPOs

TL;DR

This study uses GRMHD simulations to show that a surrounding thin disc slows down the precession of a hot, thick accretion flow, affecting the interpretation of type-C QPOs in X-ray binaries.

Contribution

It introduces the first simulation-based analysis of how a geometrically thin disc influences the precession of a misaligned hot flow.

Findings

Precession rate is reduced by 95% due to the thin disc.

Angular momentum exchange causes the slow-down of precession.

Results are relevant for understanding type-C QPOs in X-ray binaries.

Abstract

Type-C quasi-periodic oscillations (QPOs) are the low-frequency QPOs most commonly observed during the hard spectral state of X-ray binary systems. The leading model for these QPOs is the Lense-Thirring precession of a hot, geometrically thick accretion flow that is misaligned with respect to the black hole spin axis. However, none of the work done to date has accounted for the effects of a surrounding, geometrically thin disc on this precession, as would be the case in the truncated disc picture of the hard state. To address this, we perform a set of GRMHD simulations of truncated discs misaligned with the spin axes of their central black holes. Our results confirm that the inner-hot flow still undergoes precession, though at a rate that is only 5 percent of what is predicted for an isolated, precessing torus. We find that the exchange of angular momentum between the outer, thin and the inner, thick disc causes this slow-down in the precession rate and discuss its relevance to type-C QPOs.