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

Abstract

The black-hole binary Cygnus X-1 was observed for 17ks with the Suzaku X-ray observatory in 2005 October, while it was in a low/hard state with a 0.7-300keV luminosity of 4.6×10³⁷ergs^-1. The XIS and HXD spectra, spanning 0.7--400keV, were reproduced successfully, incorporating a cool accretion disk and a hot Comptonizing corona. The corona is characterized by an electron temperature of ∼100keV, and two optical depths of ∼0.4 and ∼1.5, which account for the softer and harder continua, respectively. The disk has an innermost temperature of ∼0.2keV, and is thought 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 GROJ1655-40 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 time scales of 1-200s, while the underlying disk remains unchanged.