Low/Hard State Spectra of GRO J1655 $-$ 40 Observed with Suzaku

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 $\sim$ 50mCrab at 20keV. At a distance of 3.2kpc, the 0.7-300keV luminosity was $\sim$ 5.2 $\times$ 10 $^{36}$ ergs $^{-1}$ ( $\sim$ 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 $\sim$ 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 ( $\sim$ 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 $\sim$ 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.