Observational evidence for a high-energy Compton cloud in GRO J1655-40 under a high accretion rate

Observational results on GRO J1655-40 are reported, based on the X-ray spectra obtained with the PCA and the HEXTE on-board RXTE during its 1996-1997 outburst. By referring to the 40-150 keV X-ray luminosity, L_40-150. six typical spectra were selected and analyzed. When L_40-150 is sufficiently below ∼ 5 × 10³⁶erg s^-1 (assuming a distance of 3.2 kpc and an isotropic emission), the 3-150 keV PCA/HEXTE spectra are well described by a standard spectral model consisting of an optically-thick disk spectrum and a power-law tail. When L_40-150 exceeds 5 × 10³⁶ erg s^-1, the standard model fails to reproduce the joint spectrum, due to a mild turn-over in the HEXTE spectrum. Successful fits are recovered when the power-law component is replaced by a broken power-law model with the break energy at ∼ 40 keV, or by an exponentially cutoff power-law model with the cutoff energy at 190-300 keV. The fits are further improved by employing a thermal Comptonization component, wherein the electron temperature and optical depth of the Compton cloud are estimated to be 70-85 keV and 0.45-0.65, respectively. These results provide strong evidence for the existence of thermal high-energy electrons, and reconfirm the picture of strong disk Comptonization.