X-ray spectral properties of Cyg X-1

• Published in: Advances in space research 1997, Vol.19 (1), p.5-14, Article 5
• Main Author: Ebisawa, Ken
• Format: Article
• Language: English
• ISSN: 0273-1177; 1879-1948
• DOI: 10.1016/S0273-1177(97)00030-6

Cyg X-1 is a prototype of black hole candidates with fiducial evidence of large mass. In this paper, I review X-ray spectral properties of Cyg X-1. Usually, Cyg X-1 is in the low state (hard spectral state), in which the energy spectrum in 2 – 10 keV is roughly represented with a powerlaw function, and sub-second short-term variations are significant. The continuum of the low state energy spectrum is consistent with the superposition of a power-law function with an exponential cut-off and its reflection component by an optically thick accretion disk. Thermal Comptonization of soft photons by hot plasma is a likely model for the intrinsic power-law emission. Iron K-emission lines have been reported, and study of those emission lines have a potential to verify strong gravitational fields around a black hole. However, the line emission is not prominent, as opposed to the cases of AGNs, and the line parameters are dependent on the choice of the continuum model. To determine the continuum spectral shape precisely and disentangle the iron line emission, observations having both high spectral resolution and wide energy coverage will be required. ASCA and XTE simultaneous observations may provide such a chance.