  The Galactic superluminal source GRS 1915+105 was found to experience a
peculiar X-ray variability in a narrow count rate range of the PCA detector
onboard RXTE. This can be seen as a ring-shaped pattern in the two-color
diagram of count rates, where the hard hardness F(13-40keV)/F(2-13keV) is
plotted against the soft hardness F(5-13keV)/F(2-5keV). The system runs one
cycle with periods ranging between 50 - 100 s for different observations, one
rotation in the 2-color diagram corresponding to the time between two
contiguous maxima in the light curve. We model this behaviour successfully with
the help of a self-consistent 2-phase thermal model where seed photons from an
optically thick classical disk are Comptonized in a hot spherical corona
surrounding the inner disk. In the model, changes of two parameters regulate
the paths in the 2-color diagram: the black body temperature (Tin) of the inner
disk and the Thomson optical depth multiplied by the electron temperature of
the hot phase (tau*Te). These parameters oscillate with time but with a
phase-shift between each other, causing the ring-shaped pattern. During the
observation studied in more detail (20402-01-30-00), the inner disk radius
varied with 97 s period between 20 - 35 km with an anticorrelation between the
coronal and the mass accretion rate through the disk, possibly indicating a
coupling between the disk and coronal accretion. During a typical cycle, the
inner disk radius rapidly shrinked and returned more slowly back to the
original larger value. In the rings we may see phenomena close to the black
hole horizon under near Eddington accretion rates.
