The black-hole binary Cygnus X-1 was observed for 17 ks with the Suzaku X-ray observatory in 2005 October , while it was in a low/hard state with a 0.7–300 keV luminosity of 4.6 \times 10 ^ { 37 } erg s ^ { -1 } .
The XIS and HXD spectra , spanning 0.7–400 keV , were reproduced successfully incorporating a cool accretion disk and a hot Comptonizing corona .
The corona is characterized by an electron temperature of \sim 100 keV , and two optical depths of \sim 0.4 and \sim 1.5 which account for the harder and softer continua , respectively .
The disk has the innermost temperature of \sim 0.2 keV , and is though to protrude half way into the corona .
The disk not only provides seed photons to the Compton cloud , but also produces a soft spectral excess , a mild reflection hump , and a weakly broadened iron line .
A comparison with the Suzaku data on GRO J1655 - 40 reveals several interesting spectral differences , which can mostly be attributed to inclination effects assuming that the disk has a flat geometry while the corona is grossly spherical .
An intensity-sorted spectroscopy indicates that the continuum becomes less Comptonized when the source flares up on times scales of 1–200 s , while the underlying disk remains unchanged .