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 200 keV, using data from a 10‐ks observation by the Rossi X‐ray Timing Explorer ( RXTE ). The spectrum can be well described phenomenologically by an exponentially cut‐off power law with a photon index Γ = 1.45 +0.01 −0.02 (a value considerably harder than is typically found), e‐folding energy E f  = 162 +9 −8  keV, plus a deviation from a power law that formally can be modelled as a thermal blackbody with temperature kT bb  = 1.2 +0.0 −0.1  keV. Although the 3–30 keV portion of the spectrum can be fitted 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 are included, as there is no observed softening 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 &38; Begelman, in which 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 kT c = 87 ± 5 keV, surrounded by an exterior cold disc, does provide a good description of the data (χ 2 red  = 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 these discrepancies. 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.