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 .
The spectrum can be well described phenomenologically by an exponentially cut-off power law with a photon index \Gamma = 1.45 ^ { +0.01 } _ { -0.02 } ( a value considerably harder than typically found ) , e-folding energy E _ { \mbox { \scriptsize f } } = 162 ^ { +9 } _ { -8 } keV , plus a deviation from a power law that formally can be modeled as a thermal blackbody with temperature kT _ { \mbox { \scriptsize \sc bb } } = 1.2 ^ { +0.0 } _ { -0.1 } keV .
Although the 3 – 30 keV portion of the spectrum can be fit with a reflected power law with \Gamma = 1.81 \pm 0.01 and covering fraction f = 0.35 \pm 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 [ Dove , Wilms & Begelman 1997a ] — where the temperature of the corona is determined self-consistently .
A spherical corona with a total optical depth \tau = 1.6 \pm 0.1 and an average temperature kT _ { \mbox { \scriptsize \sc c } } = 87 \pm 5 keV , surrounded by an exterior cold disc , does provide a good description of the data ( \chi ^ { 2 } _ { red } = 1.55 ) .
These models deviate from the data by up to 7 % 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 .