Observational evidence for strong disk comptonization in GRO J1655-40

An analysis was made of the multiple Rossi X-Ray Timing Explorer/Proportional Counter Array data on the candidate black hole binary system 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 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 accretion disk model (a multicolor disk model) vary significantly with time. These results reconfirm the previous report by Sobczak and coworkers. In this strong power-law state (once called the 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 power-law component becomes stronger and steeper (softer), as the disk component decreases in intensity, suggesting that some of the strong power-law emission is simply the missing optically thick disk emission. One possible explanation for this behavior is inverse Compton scattering around the disk. By refitting the same data incorporating a disk Comptonization model, the inner radius and temperature of the underlying disk are found to become more constant. These results provide one of the first observational confirmations of the scenario of 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 certain threshold, similar to 1.2-1.3 keV.