Simultaneous X-ray and gamma-ray observations of CYG X-1 in the hard state by GINGA and OSSE

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 3-30keV Ginga spectra are well represented by power laws with an energy spectral index of alpha~0.6 and a Compton reflection component including a fluorescent Fe Kalpha corresponding to the solid angle of the reflector of ~0.3x2pi. These spectra join smoothly on to the OSSE range (>=50keV) and are then cut off above ~150keV. The overall spectra can be modelled by repeated Compton scattering in a mildly relativistic, thermal plasma with an optical depth of tau~1. However, the high-energy cut-off is steeper than that due to single-temperature thermal Comptonization. It can be described by a superposition of dominant tau~1-2, thermal emission at kT~100keV and a Wien-like component from an optically thick plasma at kT~50keV. 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 disc. 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 disc-corona geometry. Rather, the observed spectra are consistent with a geometry in which the cold accretion disc (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 disc. This hot plasma consists of pure e^+/- pairs if the source size is ~5 Schwarzschild radii or it also contains protons if the size is larger.