Analysis was made of the multiple RXTE /PCA data on the promised black hole candidate with superluminal jet , GRO J 1655 - 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 \times 10 ^ { 37 } ~ { } { erg~ { } 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 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 with the power-law photon index , suggesting that the strong power-law is simply the missing optically thick disk emission .
One possible explanation for this behavior is inverse-Compton scattering in the disk .
By re-fitting the same data incorporating a disk Comptonization , 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 color temperature of the disk exceeds the certain threshold , \sim 1.2 – 1.3 keV .