Thermal pair cloud models of MeV gamma ray emission from Cygnus X-1 and the galactic center.

The HEAO-3 data for the enhanced MeV gamma ray emissions of Cygnus X-1 and the Galactic Center in 1979 can be modeled by thermal emissions from a compact pair dominated cloud surrounding the black hole. The structure of such pair clouds are derived from first principles and these results are used to constrain the mass of the central black hole, the viscosity parameter and other parameters of the innermost accretion flow. The production and escape of pairs are estimated outside the thermal cloud due to collisions of gamma rays with other gamma rays and with x rays from the disk. The annihilation of such escaped pairs in the cool medium far from the source can lead to detectable narrow 511 keV lines. These results are compared with the variable 511 keV source in the Galactic Center and the upper limits for Cygnus X-1.