X-ray dips in AGNs and microquasars – collapse time-scales of inner accretion disc

Abstract

The temporal behaviour of X-rays from some AGNs 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 time-scale of the inner accretion disc. However, estimates of this time-scale 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 time-scales for 3C 120, 3C 111, and GRS 1915+105. The Shakura–Sunyaev α parameter resulting from our model ranges from 0.02 to 0.08. Our inner disc collapse time-scale estimates agree well with those of the observed X-ray dips. We find that the collapse time-scale is most sensitive to the outer radius of the hot accretion disc.