• Takahashi, Hiromitsu
• Yamaoka, K.
• Itoh, T.
• Kubota, A.
• Group, Suzaku Swg
• Ebisawa, K.
• Yamada, S.
• Dotani, T.
• Kitamoto, S.
• Ueda, Y.
• Negoro, H.
• Makishima, K.
• Done, Chris

Abstract

We present Suzaku broad-band observational results on the Galactic black-hole binary Cygnus X-1. The source was observed for 17 ks with Suzaku in 2005 October, while it was in a low/hard state with a 0.7-300 keV luminosity of 4.6 x 10^37 erg/s. Spectra of the X-ray Imaging Spectrometer (XIS) and the Hard X-ray Detector (HXD) were clearly detected over 0.7-400 keV, and reproduced successfully incorporating a cool accretion disk and a hot Comptonizing corona. The coronal emission has a more concave spectral shape than that explained by a single Comptonizing emission model even considering the reflection component, and the corona is characterized by at least two optical depths of 0.4 and 1.5 which account for the softer and harder continua, respectively, with a common electron temperature of 100 keV. Below 1 keV, there exists significant contribution from the disk emission with the innermost temperature of 0.2 keV. The disk is thought to protrude half way into the corona, thus becoming not only the supplier of seed photons to the Compton cloud, but also the source of a soft spectral excess, a mild reflection hump, and a weakly broadened iron line. A comparison with the Suzaku data on GRO J1655-40, which was also observed in the Low/Hard state, 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....

Topics

• impedance spectroscopy
• laser emission spectroscopy
• iron