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

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.