SELF-CONSISTENT THERMAL ACCRETION DISK CORONA MODELS FOR COMPACT OBJECTS .2. APPLICATION TO CYGNUS X-1

We apply our self-consistent accretion disk corona (ADC) model, with t wo different geometries, to the broadband X-ray spectrum of the black hole candidate Cygnus X-1. As shown in a companion paper, models in wh ich the Comptonizing medium is a slab surrounding the cold accretion d isk cannot have a temperature higher than about 140 keV for optical de pths greater than 0.2, resulting in spectra that are much softer than the observed 10-30 keV spectrum of Cyg X-1. In addition, the slab-geom etry models predict a substantial ''soft excess'' at low energies, a f eature not observed for Cyg X-1, and Fe K alpha fluorescence lines tha t are stronger than observed. Previous Comptonization models in the li terature have invoked a slab geometry with optical depth tau(T) greate r than or similar to 0.3 and coronal temperature T-c similar to 150 ke V, but they are not self-consistent. Therefore, ADC models with a slab geometry are not appropriate for explaining the X-ray spectrum of Cyg X-1. Models with a spherical corona and an exterior disk, however, pr edict much higher self-consistent coronal temperatures than the slab-g eometry models. The higher coronal temperatures are due to the lower a mount of reprocessing of coronal radiation in the accretion disk, givi ng rise to a lower Compton cooling rate. Therefore, for the sphere-plu s-disk geometry, the predicted spectrum can be hard enough to describe the observed X-ray continuum of Cyg X-1 while predicting Fe fluoresce nce lines having an equivalent width of similar to 40 eV. Our best-fit parameter values for the sphere-plus-disk geometry are tau(T) approxi mate to 1.5 and T-c approximate to 90 keV.