Observational Evidence for a High-Energy Compton Cloud in GRO J1655-40 under a High Accretion Rate

• Published in: Publications of the Astronomical Society of Japan 2003-02, Vol.55 (1), p.273-279
• Main Authors: Kobayashi, Yoshihito, Kubota, Aya, Nakazawa, Kazuhiro, Takahashi, Tadayuki, Makishima, Kazuo
• Format: Article
• Language: English
• ISSN: 0004-6264; 2053-051X
• DOI: 10.1093/pasj/55.1.273

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, $L_{40 \hbox{--} 150}$ , six typical spectra were selected and analyzed. When $L_{40 \hbox{--} 150}$ is sufficiently below $\sim 5 \times 10^{36} \mathrm{erg} \,\mathrm{s}^{-1}$ (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 $L_{40 \hbox{--} 150}$ exceeds $5 \times 10^{36} \,\mathrm{erg} \,\mathrm{s}^{-1}$ , 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 $\sim 40 \,\mathrm{keV}$ , 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.