Can Lense-Thirring precession produce QPOs in supersonic accretion flows?

TL;DR

This paper argues that Lense-Thirring precession cannot produce Type C QPOs in luminous hard states of X-ray binaries because the hot inner flow's supersonic accretion speeds prevent solid-body precession.

Contribution

It demonstrates that the physical conditions in luminous hard states are incompatible with the solid-body precession model for Type C QPOs.

Findings

Hot flows in luminous states must accrete at sonic or supersonic speeds.

Supersonic accretion speeds prevent the flow from precessing as a solid body.

Solid-body precession is unlikely to explain Type C QPOs in these states.

Abstract

The timing properties of X-ray binaries are still not understood, particularly the presence of quasi-periodic oscillations (QPOs) in their X-ray power spectra. The solid-body regime of Lense-Thirring precession is one prominent model invoked to explain the most common type of QPOs, Type C. However, solid-body precession requires a specific structure that has not been examined in light of constrained properties of accretion flows. We assume in this paper, as solid-body precession requires, a disk separated into two flows at a transition radius $r_t$: a cold outer flow and a hot inner flow (playing the role of the corona). We explore the physical structure of both flows using model-independent estimates of accretion parameters. We show that, in order to reproduce the observed X-ray spectra during luminous hard states, the hot flow must accrete at sonic to supersonic speeds, unreachable with typical viscous torques. As a result of this extreme accretion speed (or high $\alpha$ parameter), no region of the disk during these states lies in the `wave-like' regime required for solid-body precession. Furthermore, we expect the flow to align with the black hole spin axis via the Bardeen-Petterson effect inside a radius $r_{\rm break}>r_t$. As a consequence, the hot inner flow cannot exhibit solid body precession -- as currently pictured in the literature -- during luminous hard states. Since Type C QPOs are prevalent in these states, we conclude that this mechanism is unlikely to be responsible for producing Type C QPOs around stellar mass black holes.