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

Abstract

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 <i>ASCA</i>, <i>RXTE</i> and <i>CGRO/OSSE</i> in 1996 May and June. The spectra consist of a dominant soft component below ∼2keV 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 <i>ASCA/RXTE</i> data yield the most probable black hole mass of M_X ≈ 10 M_⊙ and an accretion rate, Ṁ ≈ 0.5L_E/c², 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 Ka line.