Broadband spectrum of Cygnus X-1 in two spectral states with BeppoSAX

We report on the 0.5-200 keV spectral properties of Cyg X-1 observed at dif ferent epochs with the Narrow Field Instruments of the BeppoSAX satellite. The source was in its soft state during the first observation of 1996 June. In the second observation of 1996 September, the source had parameters cha racteristic to its hard state. A soft X-ray excess, a broad Fe K alpha line and Compton reflection are clearly detected in both states. The soft-state broadband continuum is well modeled by a disk blackbody (accounting for th e soft excess) and Compton upscattering of the disk photons by a hybrid, th ermal/nonthermal plasma, probably forming a corona above the disk (also giv ing rise to the Compton-reflection component). In the hard state, the prima ry hard X-ray spectrum can be well modeled by Compton upscattering of a wea k blackbody emission by a thermal plasma at a temperature of similar to 60 keV. The soft excess is then explained by thermal Comptonization of the sam e blackbody emission by another hot plasma cloud characterized by a low val ue of its Compton parameter. Finally, we find the characteristic ratio of t he bolometric flux in the soft state to that in the hard state to be about 3. This value is much more compatible with theories of state transitions th an the previously reported (and likely underestimated) value of 1.5.