Two-phase modelling of the rings in the RXTE two-color diagram of GRS   1915+105
  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. 
Osmi Vilhu and Jukka Nevalainen