Low/hard state spectra of GRO J1655-40 observed with Suzaku

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

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44 Citations (Scopus)


The Galactic black-hole binary GRO J1655-40, 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 0.7-300 keV, with an intensity of ∼ 50 mCrab at 20keV. At a distance of 3.2kpc, the 0.7-300keV luminosity was ∼ 5.2 × 1036 erg s-1 (∼ 0.7% of the Eddington luminosity for a 6 M 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 ∼ 3 keV, 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 ∼ 200 keV. The entire 0.7-300 keV 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 60-100 eV, the present XIS spectra bear no such features beyond an upper-limit equivalent width of 23 eV.

Original languageEnglish
Pages (from-to)S69-S83
JournalPublications of the Astronomical Society of Japan
Issue numberSPEC. ISS. 1
Publication statusPublished - 2008


  • Accretion disks
  • Black hole physics
  • Stars: individual (GRO J1655-40)
  • X-ray: Binaries

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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