Simultaneous X-ray and 7-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 July 6. The 3-30 keV 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 K-alpha corresponding to the solid angle of the reflector of ~0.3 times 2 Pi. These spectra join smoothly on to the OSSE range (> 50 keV) and are then cut off above ~150 keV. The overall spectra can be modelled by repeated Compton scattering in a mildly-relativistic, tau ~1, plasma. However, the high-energy cutoff is steeper than that due to single-temperature thermal Comptonisation. It can be described by either a superposition of dominant optically-thin, thermal, emission at kT~140 keV and a Wien-like component from an optically-thick plasma at kT~50 keV, or by Comptonisation by an electron distribution more sharply peaked than a Maxwellian. In all cases, 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, which rules out a disc-corona geometry. The observed spectra are consistent with a geometry in which the accretion disc only exists at large radii