Observational Evidence for Strong Disk Comptonization in Gro J 1655 − 40

Analysis was made of the multiple RXTE/PCA data on the promised black hole candidate with superluminal jet, GRO J1655−40, acquired during its 1996– 1997 outburst. The X-ray spectra can be adequately described by the sum of an optically thick disk spectrum and a power-law. When the estimated 1–100 keV power-law luminosity exceeds 1 × 10 erg s (assuming a distance of 3.2 kpc), the inner disk radius and the maximum color temperature derived from a simple accretion disk model (a multi-color disk model) vary significantly with time. These results reconfirm the previous report by Sobczak et al. (1999). In this strong power-law state (once called “very high state”), the disk luminosity decreases with temperature, in contradiction to the prediction of the standard Shakura-Sunyaev model. In the same state, the intensity of the power-law component correlates negatively with that of the disk component, and positively present address: Institute of Space and Astronautical Science, 3-1-1 Yoshinodai, Sagamihara, Kanagawa 229-8510, Japan Also Cosmic Radiation Laboratory, Institute of Physical and Chemical Research Also Universities Space Research Association