A fit to the simultaneous broadband spectrum of Cygnus X-1 using the transition disk model

We have used the transition disk model to fit the simultaneous broadband (2-500 keV) spectrum of Cygnus X-l from OSSE and Ginga observations. In this model, the spectrum is produced by saturated Comptonization within the inner region of the accretion disk, where the temperature varies rapidly with radius. In an earlier attempt, we demonstrated the viability of this model by fitting the data from EXOSAT, XMPC balloon, and OSSE observations, although these were not made simultaneously. Since the source is known to be variable, however, the results of this fit were not conclusive. In addition, since only one set of observations was used, the good agreement with the data could have been a chance occurrence. Here we improve considerably upon our earlier analysis by considering foul sets of simultaneous observations of Cygnus X-1, using an empirical model to obtain the disk temperature profile. The vertical structure is then obtained using this profile, and we show that the analysis is self-consistent. We demonstrate conclusively that the transition disk spectrum is a better fit to the observations than that predicted by the soft-photon Comptonization model. In particular, although the transition disk model has only one additional parameter, the chi(2) value is reduced and there are no systematic residuals. Since the temperature profile is obtained by fitting the data, the unknown viscosity mechanism need not be specified. The disk structure can then be used to infer the viscosity parameter alpha, which appears to vary with radius and luminosity. This behavior can be understood if alpha depends intrinsically on the local parameters such as density, height, and temperature. However, because of uncertainties in the radiative transfer, quantitative statements regarding the variation of alpha cannot yet be made.