Black hole soft X-ray transients: evolution of the cool disk and mass supply for the ADAF

Abstract

Using the black hole transient X-ray source A0620-00 as an example we study the physical interplay of three theoretical constituents for modelling these transient sources: (1) the advection-dominated accretion flow (ADAF) onto the central black hole, (2) the evaporation of matter from the cool outer disk forming a coronal flow and (3) standard disk evolution leading to outburst cycles by accretion disk instability (dwarf nova mechanism). We investigate the evolution of accretion disks during quiescence including the evaporation of gas in the inner part of the disk. About 20% of the matter is lost in a wind from the corona. The mass flow rate obtained from our model for the coronal flow towards the black hole, is about half of the matter flowing over from the companion star. It agrees with the rate independently derived from the ADAF spectral fits by Narayan et al. (1997). About one third of the matter accumulates in the outer cool disk. The computed disk evolution is consistent with the observational data from the outburst in 1975. The evolution of the accretion disk until the instability is reached shows that the critical surface density can not be reached for rates only slightly less than the rate derived here for A0620-00. Systems with such accretion rates would be globally stable, suggesting that many such faint permanently quiescent black hole X-ray binaries exist.