THE HIGH-ENERGY SPECTRUM OF CYGNUS X-1 REVISITED

The X-ray spectrum of Cyg X-1 is probably produced by thermal Comptonization in an accretion disk corona. Photons reflected from the disk are also Comptonized, so the predicted reflection spectrum is dependent on the optical depth and temperature of the scattering medium. Thus observations of reflection in the 2-20 keV energy range can constrain the electron temperature and optical depth that previously could only be derived from high-energy data. Archival EXOSAT data from Cyg X-1 indicate tau(T) approximately 0.3 and T(e) approximately 150 keV. This is perfectly compatible with recent SIGMA and OSSE observations of the high-energy spectrum, though the physical parameters derived from these experiments are very different. We show that the apparent discrepancy is due to the use of an optically thick plasma model in fitting composite spectra emitted by optically thin hotter plasmas and reflected by the underlying colder matter.