Suzaku Results on Cygnus X-1 in the Low/Hard State

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 4.6×1037 erg s. 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 ∼ 100 keV, and two optical depths of∼0.4 and∼1.5 which account for the harder and softer continua, respectively. The disk has the innermost temperature of ∼ 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.