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

AbstractThe black-hole binary Cygnus X-1 was observed for 17 ks with the Suzaku X-ray observatory in 2005October, while it was in a low/hard state with a 0.7–300 keV luminosity of 4.6×10 37 erg s −1 . The XIS andHXD spectra, spanning 0.7–400 keV, were reproduced successfully incorporating a cool accretion disk anda hot Comptonizing corona. The corona is characterized by an electron temperature of ∼100 keV, and twooptical depths of ∼0.4 and ∼1.5 which account for the harder and softer continua, respectively. The disk hasthe innermost temperature of ∼0.2 keV, and is though to protrude half way into the corona. The disk notonly provides seed photons to the Compton cloud, but also produces a soft spectral excess, a mild reflectionhump, and a weakly broadened iron line. A comparison with the Suzaku data on GRO J1655−40 revealsseveral interesting spectral differences, which can mostly be attributed to inclination effects assuming thatthe disk has a flat geometry while the corona is grossly spherical. An intensity-sorted spectroscopy indicatesthat the continuum becomes less Comptonized when the source flares up on times scales of 1–200 s, whilethe underlying disk remains unchanged.Key words: accretion disks — black hole physics — stars: individual (Cygnus X-1)— X-ray: binaries1. IntroductionLuminous soft X-ray radiation of accreting stellar-massblack holes (BHs) has generally been explained as thermalemission from optically-thick (in particular “standard”)accretion disks (Shakura & Sunyaev 1973; Makishimaet al. 1986; Dotani et al. 1997; Remillard & McClintock2006), which are expected to form around them underrather high accretion rates. In contrast, their hard X-rayproduction process is much less understood, even thoughintense hard X-ray emission characterizes black-hoe bina-ries (BHBs) among varioustypes of compact X-raysourcesin the Milky Way and Magellanic clouds.Indeed, BHBs often emit a major fraction of their ra-diative luminosity in the hard X-ray band, in the formof spectral hard-tail component if they are in so-calledhigh/soft state, or as the entire power-law (hereafter PL)like continua if they are in so-called low/hard state (here-after LHS) which appears under relatively low accretionrates. Furthermore, the hard X-ray emission (partic-ularly in the LHS) involves another interesting aspect,namely the long-known aperiodic variation over a widefrequency range (e.g., Oda et al. 1971; Oda 1977; Nolanet al. 1981; Miyamoto et al. 1991; Pottschmidt et al.2003; Remillard & McClintock 2006). These spectral andtiming studies are not limited to stellar-mass BHs, since a