Simultaneous X-ray and gamma-ray observations of Cyg X-1 in the hard state by Ginga and OSSE

• Published in: Monthly notices of the Royal Astronomical Society 1997-07, Vol.288 (4), p.958-964
• Main Authors: Gierlinski, Marek, Zdziarski, Andrzej A, Done, Chris, Johnson, W N, Ebisawa, Ken, Ueda, Yoshihiro, Haardt, Francesco, Phlips, Bernard F
• Format: Article
• Language: English
• ISSN: 0035-8711
• DOI: 10.1093/mnras/288.4.958

We present four X-ray/gamma-ray spectra of Cyg X-1 observed in the hard ('low') state simultaneously by Ginga and GRO OSSE on 1991 June 6. The four spectra have almost identical spectral form but vary in normalization within a factor of 2. The 3-30-keV Ginga spectra are well represented by power laws with an energy spectral index of 0.6 and a Compton reflection component including a fluorescent Fe K-alpha corresponding to the solid angle of the reflector of about 0.3 x 2pi. The X-ray spectra do not show the presence of the anisotropy break required if thermal Compton scattering takes place in a corona above a cold disk. Also, the flat spectral index shows that the plasma is soft-photon starved, i.e., the luminosity in incident soft X-ray seed photons is very much less than that in the hard X-rays. Furthermore, the observed solid angle of the reflector is significantly less than 2pi. These facts taken together strongly rule out a disk-corona geometry. Rather, the observed spectra are consistent with a geometry in which the cold accretion disk (which both supplies the seed soft X-rays and reflects hard X-rays) only exists at large radii, while the Comptonizing hot plasma is located in an inner region with no cold disk. (Author)