1. Low/Hard State Spectra of GRO J1655-40 Observed with Suzaku
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Takahashi, Hiromitsu, Fukazawa, Yasushi, Mizuno, Tsunefumi, Hirasawa, Ayumi, Kitamoto, Shunji, Sudoh, Keisuke, Ogita, Takayuki, Kubota, Aya, Makishima, Kazuo, Itoh, Takeshi, Parmar, Arvind N., Ebisawa, Ken, Naik, Sachindra, Dotani, Tadayasu, Kokubun, Motohide, Ohnuki, Kousuke, Takahashi, Tadayuki, Yaqoob, Tahir, Angelini, Lorella, Ueda, Yoshihiro, Yamaoka, Kazutaka, Kotani, Taro, Kawai, Nobuyuki, Namiki, Masaaki, Kohmura, Takayoshi, and Negoro, Hitoshi
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Astrophysics - Abstract
The Galactic black-hole binary GRO J1655$-$40 was observed with Suzaku on 2005 September 22--23, for a net exposure of 35 ks with the X-ray Imaging Spectrometer (XIS) and 20 ks with the Hard X-ray Detector (HXD). The source was detected over a broad and continuous energy range of 0.7--300 keV, with an intensity of $\sim$50 mCrab at 20 keV. At a distance of 3.2 kpc, the 0.7--300 keV luminosity is $ \sim 5.1 \times 10^{36}$ erg s$^{-1}$ ($\sim 0.7$ % of the Eddington luminosity for a 6 $M_{\odot}$ black hole). The source was in a typical low/hard state, exhibiting a power-law shaped continuum with a photon index of $\sim 1.6$. During the observation, the source intensity gradually decreased by 25% at energies above $\sim 3$ keV, and by 35% below 2 keV. This, together with the soft X-ray spectra taken with the XIS, suggests the presence of an independent soft component that can be represented by emission from a cool ($\sim 0.2$ keV) disk. The hard X-ray spectra obtained with the HXD reveal a high-energy spectral cutoff, with an e-folding energy of $\sim 200$ keV. Since the spectral photon index above 10 keV is harder by $\sim 0.4$ than that observed in the softer energy band, and the e-folding energy is higher than those of typical reflection humps, the entire 0.7--300 keV spectrum cannot be reproduced by a single thermal Comptonization model, even considering reflection effects. Instead, the spectrum (except the soft excess) can be successfully explained by invoking two thermal-Comptonization components with different $y$-parameters. In contrast to the high/soft state spectra of this object in which narrow iron absorption lines are detected with equivalent widths of 60--100 eV, the present XIS spectra bear no such features beyond an upper-limit equivalent width of 25 eV., Comment: 32 pages, 12 figures; accepted for publication in Publications of the Astronomical Society of Japan (PASJ)
- Published
- 2007
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