• Takahashi, Tadayuki
• Fukazawa, Yasushi
• Ueda, Yoshihiro
• Kitamoto, Shunji
• Takahashi, Hiromitsu
• Angelini, Lorella
• Kohmura, Takayoshi
• Parmar, Arvind N.
• Ebisawa, Ken
• Naik, Sachindra
• Kawai, Nobuyuki
• Kubota, Aya
• Ogita, Takayuki
• Kokubun, Motohide
• Kotani, Taro
• Yaqoob, Tahir
• Negoro, Hitoshi
• Ohnuki, Kousuke
• Yamaoka, Kazutaka
• Namiki, Masaaki
• Dotani, Tadayasu
• Itoh, Takeshi
• Mizuno, Tsunefumi
• Makishima, Kazuo
• Sudoh, Keisuke
• Hirasawa, Ayumi

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, the0.7-300keV luminosity was ̃5.2 × 10<SUP>36</SUP> ergs<SUP>-1</SUP> (̃0.7% of the Eddington luminosity for a 6M<SUB>odot</SUB> black hole). The source was in a typical low/hard state, exhibiting a power-lawshaped continuumwith 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 tothe high/soft state spectra of this object, in which narrow iron absorption lines are detectedwithequivalent widths of 60-100eV, the present XIS spectra bear no such features beyond an upper-limit equivalent width of 23eV.

Topics

• iron