The nature of spectral transitions in accreting black holes: the case of Cyg X-1

Accreting black holes radiate in one of several spectral states, switching from one to another for reasons that are as yet not understood. Using the best-studied example, Cyg X-1, we identify the geometry and physical conditions characterizing these states. In particular, we show that in the hard state most of the accretion energy is dissipated in a corona-like structure which fills the inner few tens of gravitational radii around the black hole and has Compton optical depth of order unity. In this state, an optically thick accretion disc extends out to greater distance, but penetrates only a short way into the coronal region. In the soft state, the optically thick disc moves inward and receives the majority of the dissipated energy, while the 'corona' becomes optically thin and extends around much of the inner disc. The mass-accretion rate in both states is similar to 1 x 10(-8) M. yr(-1).