Global Structure of Bimodal Accretion Disks around a Black Hole

The global structure of accretion disks around a stellar-mass black hole is examined while taking into account the energy transport by means of diffusion, as well as that by means of radial flow. It is shown that the model can have two types of solutions: In one type, the whole disk is cool and is optically thick; in the other, the disk has a hot optically thin inner region. The structure of the hot inner region of the latter is examined and an analytic formula for the location of the interface of the hot and cold region is obtained. The stability of the latter is also discussed briefly. This behavior of the model can be a possible explanation for the bimodal behavior observed in the X-ray spectra of black hole candidates.