A TRANSITION DISK MODEL FIT FOR THE BROAD-BAND X-RAY-SPECTRUM OF CYGNUS X-1

The broadband (2-500 keV) data for Cygnus X-l from observations by EXO SAT, OSSE, and the XMPC balloon, have been fitted to the transition di sk model. In this model the emission is from the inner region of an ac cretion disk where the temperature is a rapidly varying function of ra dius and the radiative mechanism is saturated Comptonization. We fit t he data to an empirical model and obtain the temperature profile that would give rise to the observed spectrum. Then we solve for the disk s tructure using this profile and show that the analysis is self-consist ent. An advantage of this method is that the viscosity mechanism need not be specified. We find that the transition model spectrum seems to be a better fit compared to a power law with exponential cutoff. In pa rticular, a second component (with peak around 100 keV) that has been used in the past to explain the spectrum is not required here. We emph asize the need to conduct simultaneous broadband observations of this source in order to test ideas such as those presented here.