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 transition from the hard to the soft state and in the soft state by ASCA, RXTE and CGRO/OSSE in 1996 May and June. The spectra consist of a dominant soft component below similar to 2keV and a power-law-like continuum extending to at least similar to 800 keV. We interpret them as emission from an optically thick, cold accretion disc and from an optically thin, non-thermal corona above 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 into account the torque-free inner-boundary condition. If the disc extends down 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 rate, (M) over dot approximate to 0.5L(E)/c(2), locating Cyg X-l in the soft state in the upper part of the stable, gas-pressure-dominated, accretion-disc 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-thermal distribution. The electron distribution can be characterized by a Maxwellian with an equilibrium temperature of kT(e) similar to 30-50 keV, a Thomson optical depth of tau similar to 0.3 and a quasi-power-law tail, The compactness of the corona is 2 less than or similar to l(h) less than or similar to 7, and a presence of a significant population of electron-positron pairs is ruled out, We find strong signatures of Compton reflection from a cold and ionized medium, 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.