Broadband Spectrum of Cygnus X‐1 in Two Spectral States withBeppoSAX

We report on the 0.5--200 keV spectral properties of Cyg X-1 observed atdifferent epochs with the Narrow Field Instruments of the BeppoSAX satellite.The source was in its soft state during the first observation of 1996 June. Inthe second observation of 1996 September, the source had parameterscharacteristic 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-statebroad-band continuum is well modeled by a disk blackbody (accounting for thesoft excess) and Compton upscattering of the disk photons by a hybrid,thermal/non-thermal, plasma, probably forming a corona above the disk (alsogiving rise to the Compton-reflection component). In the hard state, theprimary hard X-ray spectrum can be well modeled by Compton upscattering of aweak blackbody emission by a thermal plasma at a temperature of $\sim 60$ keV.The soft excess is then explained by thermal Comptonization of the sameblackbody emission by another hot plasma cloud characterized by a low value ofits Compton parameter. Finally, we find the characteristic ratio of thebolometric flux in the soft state to that in the hard state to be about 3. Thisvalue is much more compatible with theories of state transitions than thepreviously reported (and likely underestimated) value of 1.5.Comment: 15 pages, 5 figures. Accepted for publication in Ap