X-ray Dips in AGN and Microquasars -- Collapse Timescales of Inner Accretion Disc

TL;DR

This paper estimates the collapse timescales of inner accretion discs in AGN and microquasars using a viscosity model based on cosmic ray diffusion, aligning well with observed X-ray dip durations.

Contribution

It introduces a new viscosity prescription derived from cosmic ray diffusion simulations to model hot accretion disc collapse timescales.

Findings

Collapse timescales match observed X-ray dips

Viscosity parameter α ranges from 0.02 to 0.08

Collapse timescale is sensitive to outer disc radius

Abstract

The temporal behaviour of X-rays from some AGN and microquasars is thought to arise from the rapid collapse of the hot, inner parts of their accretion discs. The collapse can occur over the radial infall timescale of the inner accretion disc. However, estimates of this timescale are hindered by a lack of knowledge of the operative viscosity in the collisionless plasma comprising the inner disc. We use published simulation results for cosmic ray diffusion through turbulent magnetic fields to arrive at a viscosity prescription appropriate to hot accretion discs. We construct simplified disc models using this viscosity prescription and estimate disc collapse timescales for 3C 120, 3C 111, and GRS 1915+105. The Shakura-Sunyaev {\alpha} parameter resulting from our model ranges from 0.02 to 0.08. Our inner disc collapse timescale estimates agree well with those of the observed X-ray dips. We find that the collapse timescale is most sensitive to the outer radius of the hot accretion disc.