Marek Gierlinski1, Andrzej A. Zdziarski2, Chris Done3, W. Neil Johnson4, Ken Ebisawa5, Joshihiro Ueda6, Francesco Haardt7, Bernard F. Phlips4
1Astronomical Observatory, Jagiellonian University,
Orla 171, 30-244 Cracow, Poland
2N. Copernicus Astronomical Center, Bartycka 18,
00-716 Warsaw, Poland
3Department of Physics, University of Durham, Durham
DH1 3LE, UK
4E. O. Hulburt Center for Space Research, Naval
Research Laboratory, Washington, DC 20375, USA
5Laboratory for High Energy Astrophysics, NASA/GSFC,
Greenbelt, MD 20771, USA
6Institute of Space and Astronautical Science, 3-1-1,
Yoshinodai, Sagamihara-shi, Kanagawa 229, Japan
7Department of Astronomy and Astrophysics, Gothenburg
University, 41296 Gothenburg, Sweden
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 four spectra have almost identical spectral form but vary in the normalisation within a factor of two. 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 kTe ~140 keV and a Wien-like component from an optically-thick plasma at kTe ~50 keV, or by Comptonisation by an electron distribution more sharply peaked than a Maxwellian. For the latter case, we find that a power-law electron distribution between the Lorentz factors of ~1 and ~2 can qualitatively explain the observed spectra. 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 (which both supplies the seed soft X-rays and reflects hard X-rays) only exists at large radii, while the particle acceleration takes place in an inner disc-free region. This hot plasma consists of either e+e- pairs if the source size is less than ~2 Schwarzschild radii or electrons and protons if the size is larger.