STRUCTURE OF THERMAL PAIR CLOUDS AROUND GAMMA-RAY-EMITTING BLACK-HOLES

Using certain simplifying assumptions, we derive from first principles the general structure of a quasi-spherical thermal pair-balanced cloud surrounding an accreting black hole. We find that pair-dominated hot solutions exist only for a restricted range of the viscosity parameter. These results are then applied as examples to the 1979 HEAO 3 gamma-ray data of Cygnus X-1 and the Galactic center. We obtain values for the viscosity parameter alpha-BAR lying in the range approximately 0.1-0.01. Since the lack of synchrotron soft photons requires the magnetic field to be typically less than 1% of the equipartition value, we conclude that magnetic field cannot be the main contributor to the viscous stress of the inner accretion flow, at least during the high gamma-ray states.