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 3-30 keV Ginga spectra are well represented by power laws with an energy spectral index of alpha similar to 0.6 and a Compton reflection component including a fluorescent Fe K alpha corresponding to the solid angle of the reflector of similar to 0.3 x 2 pi. These spectra join smoothly on to the OSSE range (greater than or equal to 50 keV) and are then cut off above similar to 150 keV. The overall spectra can be modelled by repeated Compton scattering in a mildly relativistic, thermal plasma with an optical depth of tau similar to 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 similar to 1-2, thermal emission at kT similar to 100 keV and a Wien-like component from an optically thick plasma at kT similar to 50 keV. 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 2 pi. 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 similar to 5 Schwarzschild radii or it also contains protons if the size is larger.