A FIT TO THE SIMULTANEOUS BROAD-BAND SPECTRUM OF CYGNUS X-1 USING THETRANSITION DISK MODEL

We have used the transition disk model to fit the simultaneous broadba nd (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 v aries 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. S ince the source is known to be variable, however, the results of this fit were not conclusive. In addition, since only one set of observatio ns 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, usi ng an empirical model to obtain the disk temperature profile. The vert ical structure is then obtained using this profile, and we show that t he analysis is self-consistent. We demonstrate conclusively that the t ransition disk spectrum is a better fit to the observations than that predicted by the soft-photon Comptonization model. In particular, alth ough the transition disk model has only one additional parameter, the chi(2) value is reduced and there are no systematic residuals. Since t he temperature profile is obtained by fitting the data, the unknown vi scosity mechanism need not be specified. The disk structure can then b e used to infer the viscosity parameter alpha, which appears to vary w ith radius and luminosity. This behavior can be understood if alpha de pends intrinsically on the local parameters such as density, height, a nd temperature. However, because of uncertainties in the radiative tra nsfer, quantitative statements regarding the variation of alpha cannot yet be made.