BROAD-BAND X-RAY-SPECTRUM OF CYGNUS X-1

We present the hard X-ray (20 - 100 keV) observations of Cygnus X-1 ob tained using a large area balloon-borne Xenon filled Multi-anode Propo rtional Counter (XMPC) telescope. The observations were carried out du ring the gamma(2) state of the source and we obtain a power law photon index of 1.62+/-0.07. To constrain the spectral shape of the source, we have analyzed the archival EXOSAT ME argon and GSPC data in the low energies (2 - 20 keV band) as well as the archival OSSE data in the h igh energies (50 - 500 keV). The data in different energy bands are no t obtained in simultaneous observations, but they pertain to the gamma (2) state of the source. We have attempted a combined fit to the wide band data using appropriate mutual detector calibrations. This method implicitly assumes that the variations in the source intensity in the gamma(2) State is mainly due to the variations in the normalisations o f the spectral components rather than any change in the spectral param eters. A combined fit to the EXOSAT and XMPC data (2 - 100 keV) shows that the observed spectrum requires a low energy absorption correspond ing to the Galactic interstellar absorption, a low energy excess model ed as a blackbody, a narrow emission line due to iron K-alpha and a co ntinuum. The continuum can be either modeled as a power law with a ref lection bump or a Comptonisation model with an additional bump which c an be modeled as the partial covering with a heavy absorber. To resolv e between these two models, we have attempted a combined fit to the 2 - 500 keV data obtained from EXOSAT, XMPC and OSSE. We find that a sin gle Comptonisation model cannot adequately represent the continuum. Th e observed excess is at higher energies (about 100 keV) and it cannot be modeled as reflection of power law or Comptonisation model. We find that a two component Comptonisation model adequately represents the d ata. We explore the possible emission region that is responsible for t he observed spectrum.