Rapid State Transitions in the Galactic Black Hole Candidate Source GRS   1915+105
  We examine the X-ray spectral and temporal behavior of the Galactic black hole candidate source GRS 1915+105 during its spectral state transition observed in 1997 March - August. The source was making a slow transition from a low-hard state to a high-soft state in about three months and during this transition it was exhibiting a series of fast variations which can be classified as bursts. During one type of bursts, called the irregular bursts, it was found that the source makes rapid transitions between two intensity states. We have analyzed the RXTE PCA data obtained on 1997 June 18 (when the source was making rapid state transitions) and compared the results with two sets of data each pertaining to the low hard-state and high-soft states. We find that the power density spectrum (PDS) during the burst shows remarkable similarity to that seen during the high-soft state of the source. The PDS during the quiescence, on the other hand, is quite flat and is very similar to the PDS commonly seen during the canonical low-hard state of black hole X-ray binaries. A low frequency QPO at $\sim$3 Hz is observed during the quiescence, a property seen during the low-hard state of the source. The X-ray spectrum of the source during quiescence is similar to the spectrum seen during the low-hard state of the source and the spectra during the burst and high-soft state are very similar to each other. These observations corroborate the suggestion made by Yadav et al. (1999) that during the irregular bursts the source shows spectral state variations at very fast time scales. We explain such fast state changes using the two component accretion flow model in which the accretion disk consists of a geometrically thin (and optically thick) standard disk co-existing with an advection dominated hot thick disk. 
A. R. Rao, J. S. Yadav and B. Paul