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, switc hing from one to another for reasons that are as yet not understood. U sing the best-studied example, Cyg X-1, we identify the geometry and p hysical 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 gravitationa l radii around the black hole and has Compton optical depth of order u nity. In this state, an optically thick accretion disc extends out to greater distance, but penetrates only a short way into the coronal reg ion. In the soft state, the optically thick disc moves inward and rece ives the majority of the dissipated energy, while the 'corona' becomes optically thin and extends around much of the inner disc. The mass-ac cretion rate in both states is similar to 1 x 10(-8) M. yr(-1).