Low/Hard State Spectra of GRO J1655-40 Observed with Suzaku
Dotani, Tadayasu; Yaqoob, Tahir; Yamaoka, Kazutaka; Fukazawa, Yasushi; Kokubun, Motohide; Makishima, Kazuo; Takahashi, Tadayuki; Ueda, Yoshihiro; Naik, Sachindra; Ebisawa, Ken; Takahashi, Hiromitsu; Kubota, Aya; Namiki, Masaaki; Itoh, Takeshi; Angelini, Lorella; Kawai, Nobuyuki; Kitamoto, Shunji; Kohmura, Takayoshi; Mizuno, Tsunefumi; Ohnuki, Kousuke; Hirasawa, Ayumi; Negoro, Hitoshi; Kotani, Taro; Sudoh, Keisuke; Ogita, Takayuki; Parmar, Arvind N.
Japan, Netherlands, United States
Abstract
The Galactic black-hole binary GROJ1655-40, a source harboring superluminal jets, was observed with Suzaku on 2005 September 22-23. The source was detected over a broad and continuous energy range of 0.7-300keV, with an intensity of ∼50mCrab at 20keV. At a distance of 3.2kpc, the 0.7-300keV luminosity was ∼5.2 × 1036 ergs-1 (∼0.7% of the Eddington luminosity for a 6Modot black hole). The source was in a typical low/hard state, exhibiting a power-law shaped continuum with a photon index of ∼1.6. During the observation, the source intensity gradually decreased by 25% at energies above ∼3keV, and by 35% below 2keV. This, together with the soft X-ray spectra taken with the X-ray Imaging Spectrometer (XIS), suggests the presence of an independent soft component that can be represented by emission from a cool (∼0.2keV) disk. The hard X-ray spectra obtained with the Hard X-ray Detector reveal a high-energy spectral cutoff, with an e-folding energy of ∼200keV. The entire 0.7-300keV spectrum cannot be reproduced by a single thermal Comptonization model, even when 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 60-100eV, the present XIS spectra bear no such features beyond an upper-limit equivalent width of 23eV.