Hard X-ray spectrum of CYG X-1

The X-ray spectrum and luminosity of the black hole candidate Cyg X-1 are discussed and compared with theoretical predictions of emission from hot accretion disks. Numerous rocket, satellite and balloon measurements have shown most Cyg X-1 spectra to be characterized by a power law. The X-ray emission has been explained in terms of disk accretion onto a black hole; hard X rays are mainly produced by Comptonization by hot gas of low energy photons produced in cold zones of an unstable disk. Theoretical calculations of the Comptonization of soft photons by hot gas are found to agree well with the low state spectrum observed from Cyg X-1. The accretion rate, derived from the observed luminosity, however, is insufficient to obtain an unstable disk and possible explanations for this discrepancy are offered.