Broad band X-ray spectrum of Cygnus X-1

We present the hard X-ray (20-100 keV) observations of Cygnus X-1 obtained using a large area balloon-borne Xenon filled Multi-anode Proportional Counter (XMPC) telescope. The observations were carried out during 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 high energies (50-500 keV). The data in different energy bands are not 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. A combined fit to the EXOSAT and XMPC data (2-100 keV) shows that the observed spectrum requires a low energy absorption corresponding to the Galactic interstellar absorption, a low energy excess modeled as blackbody, a narrow emission line due to iron K_alpha and a continuum. The continuum can be either modeled as a power law with a reflection bump or a Comptonisation model with an additional bump which can be modeled as the partial covering with a heavy absorber. To resolve between these two models, we have attempted combined fit to the 2-500 keV data obtained from EXOSAT, XMPC and OSSE. We find that two component Comptonisation model adequately represents the data. We explore the possible emission region that is responsible for the observed spectrum.