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M.L. McConnell, J.M. Ryan (UNH), W. Collmar, V. Sch\"onfelder, H. Steinle, A.W. Strong (MPE), H. Bloemen, W. Hermsen, L. Kuiper (SRON-Utrecht), K. Bennett (ESA-SSD), B. Phlips (NRL), J. Ling (JPL/NASA-HQ)
We have used data from the first three years of the CGRO mission to assemble a broad-band \gamma-ray spectrum of the galactic black hole candidate Cygnus X-1. Contemporaneous data from the COMPTEL, OSSE and BATSE experiments on CGRO have been further selected on the basis of the hard X-ray flux measured by BATSE. The resulting spectrum provides a consistent picture of the emission in the low energy \gamma-ray range above 100 keV. There is clear evidence for emission out to at least 2 MeV and perhaps as high as 5 MeV. Comparison with standard Comptonization models shows evidence for substantial hardening of the spectrum at energies above a few hundred keV. The level of spectral hardening seen in these data is well below that previously reported from some earlier satellite and balloon experiments, but is nonetheless very significant. Given the high sensitivity of the COMPTEL data in the MeV range, this spectrum will provide a useful comparison for the various models of the high energy emission that seek to explain the deviations from the standard Comptonization models.
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