Thermal Comptonization in GRS 1915+105

The Rossi X-ray Timing Explorer data of GRS 1915+105 from several observingperiods are modeled with a thermal Comptonization model. Best-fit modelsindicate that there is a strong correlation between the inner disk temperatureand the disk luminosity. The hard Comptonized luminosity does not dependsignificantly on the total luminosity. The spectral hardness of the Comptonizedradiation, the fraction of seed soft photons scattered by the Comptonizingcloud, its Thomson optical depth, and the fraction of the total powerdissipated in the optically thin hot plasma, all strongly anticorrelate withthe luminosity. We find that the inner disk radius is almost constant and thatthe hot Comptonizing corona shrinks at high luminosities. We note that the fitsusing xspec diskbb + power law model underestimate the amplitude of theblackbody component (and therefore the corresponding size of the emittingregion) and overestimate the absorption column density and the total, correctedfor absorption, luminosity.Comment: 4 pages in emulateapj5.sty, 3 figures. Substantially revised version accepted for publication in ApJ Letter