RXTE OBSERVATION OF CYGNUS-X-1 - I - SPECTRAL-ANALYSIS

We present the results of the analysis of the broad-band spectrum of C ygnus X-1 from 3.0 to 200 keV, using data from a 10-ks observation by the Rossi X-ray Timing Explorer (RXTE). The spectrum can be well descr ibed phenomenologically by an exponentially cut-off power law with a p hoton index Gamma = 1.45(-0.02)(+0.01) (a value considerably harder th an is typically found), e-folding energy E-f = 162(-8)(+9) keV, plus a deviation from a power law that formally can be modelled as a thermal blackbody with temperature kT(BB) = 1.2(-0.1)(+0.0) keV, Although the 3-30 keV portion of the spectrum can be fitted with a reflected power law with Gamma = 1.81 +/- 0.01 and covering fraction f = 0.35 +/- 0.0 2, the quality of the fit is significantly reduced when the HEXTE data in the 30-100 keV range are included, as there is no observed softeni ng in the power law within this energy range. As a physical descriptio n of this system, we apply the accretion disc corona models of Dove, W ilms & Begelman, in which the temperature of the corona is determined self-consistently. A spherical corona with a total optical depth tau = 1.6 +/- 0.1 and an average temperature kT(C) = 87 +/- 5 keV, surround ed by an exterior cold disc, does provide a good description of the da ta (chi(red)(2) = 1.55). These models deviate from the data by up to 7 per cent in the 5-10 keV range, and we discuss possible reasons for t hese discrepancies. However, considering how successfully the spherica l corona reproduces the 10-200 keV data, such 'photon-starved' coronal geometries seem very promising for explaining the accretion processes of Cygnus X-1.