The emission region of Cygnus X-1 MeV gamma rays.

The thermal pair cloud model of Liang and Dermer (1988) for the HEAO-3 Cygnus X-1 gamma ray bump requires the lack of soft photons, either external or internal, other than self-emitted bremsstrahlung. The self-consistency of this requirement is addressed, and it is shown that the soft photons impinging from the X-ray disk can be effectively shielded by a transition region. The pair cloud model also makes it possible to put stringent bounds on the mass of the black hole, Keplerianess of the accretion flow near the innermost stable orbit, and the average viscosity from first principles.