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X.L. Zhang, S.N. Zhang (UAH), H.G. Xu (SJTU/China), X.J. Sun, Y.S. Yao (UAH), W. Cui (MIT), W. Chen (GSFC/UMd)
The results of spectral modeling of the data for a series of RXTE observations of GRO J1655-40 are presented. The thermal Comptonization model is used instead of the power-law model for the hard component of the two-component continuum spectra. This is because that it is commonly believed that the photons radiated from the inner disk area are Comptonized in a hot and optically thin corona. Photons scattered by hot electrons compose the hard component; those escaped from the corona directly form the soft component. It is known that the luminosity of the emission from the inner disk area is proportional to the inner disk radius squared. However, due to the Comptonization process in the corona, the observed soft component can no longer be used to determine the inner disk radius directly. The previously reported dramatic variation of the apparent inner disk radius of GRO J1655-40 during its outburst may be a consequence of this effect. A correction should be made in order to get the original spectra of the radiation. We used this strategy to analyze the data for GRO J1655-40, and obtained a more stable inner disk radius.
Another factor in determining the inner disk radius is that the detected soft component (even after the above correction) is in fact not the same as the blackbody emission from the cold disk, due to saturated electron scattering of the original blackbody photons in the disk. Therefore a color correction must be made before the physical inner disk radius is calculated. We have developed a procedure in determining the color correction factor from observational data. With the ASCA data of GRO J1655-40 we found that the color correction factor may vary significantly between different observations and the finally corrected physical inner disk radius remains remarkably stable over a large range of luminosity and spectral states.