Radiation mechanisms and geometry of Cygnus X-1 in the soft state

We present X-ray/gamma-ray spectra of Cyg X-1 observed during the transitio n from the hard to the soft state and in the soft state by ASCA, RXTE and C GRO/OSSE in 1996 May and June. The spectra consist of a dominant soft compo nent below similar to 2keV and a power-law-like continuum extending to at l east similar to 800 keV. We interpret them as emission from an optically th ick, cold accretion disc and from an optically thin, non-thermal corona abo ve the disc, A fraction f greater than or similar to 0.5 of total available power is dissipated in the corona, We model the soft component by multicolour blackbody disc emission taking i nto account the torque-free inner-boundary condition. If the disc extends d own to the minimum stable orbit, the ASCA/RXTE data yield the most probable black hole mass of M-X approximate to 10 M-circle dot and an accretion rat e, (M) over dot approximate to 0.5L(E)/c(2), locating Cyg X-l in the soft s tate in the upper part of the stable, gas-pressure-dominated, accretion-dis c solution branch. The spectrum of the corona is well modelled by repeated Compton scattering of seed photons from the disc off electrons with a hybrid, thermal/non-ther mal distribution. The electron distribution can be characterized by a Maxwe llian with an equilibrium temperature of kT(e) similar to 30-50 keV, a Thom son optical depth of tau similar to 0.3 and a quasi-power-law tail, The com pactness of the corona is 2 less than or similar to l(h) less than or simil ar to 7, and a presence of a significant population of electron-positron pa irs is ruled out, We find strong signatures of Compton reflection from a cold and ionized med ium, presumably an accretion disc, with an apparent reflector solid angle, Omega/2 pi similar to 0.5-0.7. The reflected continuum is accompanied by a broad iron K alpha line.