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The Galactic superluminal source GRS 1915+105 was found to experience apeculiar X-ray variability in a narrow count rate range of the PCA detectoronboard RXTE. This can be seen as a ring-shaped pattern in the two-colordiagram of count rates, where the hard hardness F(13-40keV)/F(2-13keV) isplotted against the soft hardness F(5-13keV)/F(2-5keV). The system runs onecycle with periods ranging between 50 - 100 s for different observations, onerotation in the 2-color diagram corresponding to the time between twocontiguous maxima in the light curve. We model this behaviour successfully withthe help of a self-consistent 2-phase thermal model where seed photons from anoptically thick classical disk are Comptonized in a hot spherical coronasurrounding the inner disk. In the model, changes of two parameters regulatethe paths in the 2-color diagram: the black body temperature (Tin) of the innerdisk and the Thomson optical depth multiplied by the electron temperature ofthe hot phase (tau*Te). These parameters oscillate with time but with aphase-shift between each other, causing the ring-shaped pattern. During theobservation studied in more detail (20402-01-30-00), the inner disk radiusvaried with 97 s period between 20 - 35 km with an anticorrelation between thecoronal and the mass accretion rate through the disk, possibly indicating acoupling between the disk and coronal accretion. During a typical cycle, theinner disk radius rapidly shrinked and returned more slowly back to theoriginal larger value. In the rings we may see phenomena close to the blackhole horizon under near Eddington accretion rates.Comment: ApJ Lett in pres

Two-Phase Modeling of the Rings in the [ITAL]RXTE[/ITAL] Two-Color Diagram of GRS 1915+105