A transition disk model for cygnus X-1

The standard cold accretion disk theory for black hole systems such as Cygnus X-1 is not self-consistent when the effective optical depth is less than unity. In this paper, we develop a scheme for solving the disk structure that is valid at all optical depths. We show that the disk typically consists of three regions: (a) an outer cold disk, (b) a middle transition zone, and (c) an inner two-temperature disk. The emergent spectrum from the transition region is due primarily to a sum of Wien peaks instead of the soft-photon Comptonization within a geometrically thick, hot inner region as proposed in earlier models, and it matches well with the observed X-ray power-law continuum from these sources. In the picture developed here, we include the possibility of the presence of a magnetic field having an equipartition pressure with the plasma. We suggest that variations in this magnetic field and the corresponding viscosity parameter a can account for the two long-term X-ray states of Cygnus X-1. The transition disk region is subject to both thermal and secular instabilities, but it appears that these do not transform the disk into a two-temperature configuration (Taam & Lin 1984). Instead, we speculate that these instabilities give rise to structural fluctuations which we observe as quasi-periodic oscillations.