The Spectral States of Black Hole X-ray Binary Sources
  A framework for the interpretation of the spectral states of black hole X-ray transients based on the diversity of accretion disk models is introduced. Depending on the mass accretion rate, it is proposed that the accretion disk is described by one or a combination of the following structures: optically thick disk, advection-dominated disk, corona-disk, and non-steady state disk. In particular, it is suggested that the very high, high, low, and off states are characterized by mass accretion rates of decreasing magnitude. The very high state corresponds to mass accretion rates near the Eddington limit in which an optically thin non steady inner region is surrounded by an optically thick structure. In the high state, the inner region is optically thin and advection-dominated or optically thick. The low hard state is interpreted in terms of a disk-corona system and the off state in terms of an optically thin disk dominated by advective energy transport into the black hole. The possible observational consequences of such a paradigm are discussed. 
Xingming Chen and Ronald E. Taam