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 \sim 1 \times 10 ^ { -8 } M _ { \odot } yr ^ { -1 } .