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Radiation mechanisms and geometry of Cygnus X-1 in the soft state |
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| Authors: | Marek Gierli&#;ski rzej A Zdziarski Juri Poutanen Paolo S Coppi Ken Ebisawa W Neil Johnson |
| Institution: | Astronomical Observatory, Jagiellonian University, Orla 171, 30-244 Cracow, Poland; N. Copernicus Astronomical Center, Bartycka 18, 00-716 Warsaw, Poland; Stockholm Observatory, SE-133 36 Saltsjöbaden, Sweden; Astronomy Department, Yale University, PO Box 208101, New Haven, CT 062520-8101, USA; Code 660.2, Laboratory for High Energy Astrophysics, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA (also at Universities Space Research Association); E. O. Hulburt Center for Space Research, Naval Research Laboratory, Washington, DC 20375, USA |
| 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 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 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 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. |
| Keywords: | accretion accretion discs radiation mechanisms: non-thermal stars: individual: Cygnus X-1 gamma-rays: observations gamma-rays: theory X-rays: stars |