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RXTE observation of Cygnus X-1: Spectral analysis

Abstract 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 −0.02 +0.01 , e-folding energy E f = 162 −8 +9 keV, plus a deviation from a power law that formally can be modeled as a thermal blackbody with temperature kT BB = 1.2 −0.1 +0.0 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 & Begelman [1]. 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 kT C = 87 ± 5 keV, surrounded by an exterior cold disc, does provide a good description of the data ( χ red 2 = 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.