RXTE Observation of Cygnus X-1: I. Spectral Analysis

We present the results of the analysis of the broad-band spectrum of Cygnus X-1 from 3.0 to 200keV, using data from a 10ksec observation by the Rossi X-ray Timing Explorer. The spectrum can be well described phenomenologically by an exponentially cut-off power law with a photon index Γ = 1.45 −0.02 (a value considerably harder than typically found), 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 −0.1 keV. Although the 3–30 keV portion of the spectrum can be fit with a reflected power law with Γ = 1.81 ± 0.01 and covering fraction f = 0.35 ± 0.02, the quality of the fit is significantly reduced when the HEXTE data in the 30–100 keV range is included, as there is no observed hardening in the power law within this energy range. As a physical description of this system, we apply the accretion disc corona models of Dove, Wilms & Begelman (1997a) — where the temperature of the corona is determined self-consistently. 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 5−10keV range, and we discuss possible reasons for these discrepancies. However, considering how successfully the spherical corona reproduces the 10−200keV data, such “photon-starved” coronal geometries seem very promising for explaining the accretion processes of Cygnus X-1.