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 γ 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 γ 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 γ 2 state is mainly due to the variations in the normalisations of the spectral components rather than any change in the spectral parameters. 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 a blackbody, a narrow emission line due to iron K α and a continuum. The continuum can be either modeled as a power law with a reflection bump or a Comptoni-sation 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 a combined fit to the 2 − 500 keV data obtained from EXOSAT, XMPC and OSSE. We find that a single Comptonisation model cannot adequately represent the continuum. The 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 data. We explore the possible emission region that is responsible for the observed spectrum.