A transition disk model fit for the broadband X-ray spectrum of Cygnus X-1

The broadband (2-500 keV) data for Cygnus X-l from observations by EXOSAT, OSSE, and the XMPC balloon, have been fitted to the transition disk model. In this model the emission is from the inner region of an accretion disk where the temperature is a rapidly varying function of radius and the radiative mechanism is saturated Comptonization. We fit the data to an empirical model and obtain the temperature profile that would give rise to the observed spectrum. Then we solve for the disk structure using this profile and show that the analysis is self-consistent. 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 particular, a second component (with peak around 100 keV) that has been used in the past to explain the spectrum is not required here. We emphasize the need to conduct simultaneous broadband observations of this source in order to test ideas such as those presented here.