RXTE observation of Cygnus X-1: Spectral analysis

We present the results of the analysis of the broad-band spectrum of Cygnus X-1 from 3.0 to 200 keV, using data from a 10 ksec observation by the Rossi X-ray Timing Explorer. Although the spectrum can be well described phenomenologically by an exponentially cut-off power law (photon index Γ = 1.45+0.01-0.02, e-folding energy Ef = 162+9-8 keV, plus a deviation from a power law that formally can be modeled as a thermal blackbody with temperature kTBB = 1.2+0.0-0.1 keV), the inclusion of a reflection component does not improve the fit. As a physical description of this system, we apply the accretion disc corona (ADC0 models of Dove, Wilms and Begelman. A slab-geometry ADC model is unable to describe the data. However, a spherical corona, with a total optical depth τ = 1.6 ± 0.1 and an average temperature kTC = 87 ± 5 keV, surrounded by an exterior cold disc, does provide a good description of the data (χ2red 1.55). These models deviate from the data by up to 7% in the 5-10 keV range. However, considering how successfully the spherical corona reproduces the 10-200 keV data, such 'photon-starved' coronal geometries seem very promising for explaining the accretion processes of Cygnus X-1