The Galactic black-hole binary GRO J1655 - 40 , a source harboring superluminal jets , was observed with Suzaku on 2005 September 22–23 , for a total time span of \sim 1 day , and 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 can not 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 .