Observational evidence for strong disk comptonization in GRO J1655-40

An analysis was made of the multiple Rossi X-Ray Timing Explorer/Proportion al Counter Array data on the candidate black hole binary system with superl uminal 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 x 10(37) ergs s(-1) (assuming a distance of 3.2 kpc), the inner disk radius and the maximum color temperature derived from a simple accreti on disk model (a multicolor disk model) vary significantly with time. These results reconfirm the previous report by Sobczak and coworkers. In this st rong power-law state (once called the "very high state"), the disk luminosi ty decreases with temperature, in contradiction to the prediction of the st andard Shakura-Sunyaev model. In the same state, the power-law component be comes stronger and steeper (softer), as the disk component decreases in int ensity, suggesting that some of the strong power-law emission is simply the missing optically thick disk emission. One possible explanation for this b ehavior is inverse Compton scattering around the disk. By refitting the sam e data incorporating a disk Comptonization model, the inner radius and temp erature of the underlying disk are found to become more constant. These res ults provide one of the first observational confirmations of the scenario o f disk Comptonization in the strong power-law state. This strong power-law state seems to appear when the color temperature of the disk exceeds a cert ain threshold, similar to 1.2-1.3 keV.