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

We present X‐ray/ γ ‐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 ∼2 keV and a power‐law‐like continuum extending to at least ∼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 ≳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 ≈10 M ⊙ and an accretion rate, , locating Cyg X‐1 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 ∼30–50 keV, a Thomson optical depth of τ ∼0.3 and a quasi‐power‐law tail. The compactness of the corona is 2≲ℓ h ≲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, Ω/2π∼0.5–0.7. The reflected continuum is accompanied by a broad iron K α line.