Broadband Spectrum of Cygnus X‐1 in Two Spectral States with<i>BeppoSAX</i>

Abstract

We report on the 0.5200 keV spectral properties of Cyg X-1 observed at di erent epochs with the Narrow Field Instruments of the BeppoSAX satellite. The source was in its soft state during the rst observation of 1996 June. In the second observation of 1996 September, the source had parameters characteristic to its hard state. A soft X-ray excess, a broad Fe Ka line and Compton reection are clearly detected in both states. The soft-state broadband continuum is well modeled by a disk blackbody (accounting for the soft excess) and Compton upscattering of the disk photons by a hybrid, thermal/ nonthermal plasma, probably forming a corona above the disk (also giving rise to the Compton-reection component). In the hard state, the primary hard X-ray spectrum can be well modeled by Compton upscattering of a weak blackbody emission by a thermal plasma at a temperature of D60 keV. The soft excess is then explained by thermal Comptonization of the same blackbody emission by another hot plasma cloud characterized by a low value of its Compton parameter. Finally, we nd the characteristic ratio of the bolometric ux 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 than the previously reported (and likely underestimated) value of 1.5.