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 h ole candidate source GRS 1915+105 during its spectral state transition obse rved in 1997 March-August. The source was making a slow transition from a l ow hard state to a high soft state in about 3 months, and during this trans ition it was exhibiting a series of fast variations, which can be classifie d as bursts. During one type of burst, called the irregular burst, it was f ound that the source makes rapid transitions between two intensity states. We have analyzed the RXTE PCA data obtained on 1997 June 18 (when the sourc e was making rapid state transitions) and compared the results with two set s of data each pertaining to the low hard state and high soft states. We fi nd 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 d uring 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 quasi-periodic oscillation at similar to3 H z is observed during the quiescence, a property seen during the low hard st ate of the source. The X-ray spectrum of the source during quiescence is si milar 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 othe r. These observations corroborate the suggestion made by Yadav and coworker s that during the irregular bursts the source shows spectral state variatio ns at very fast timescales. We explain such fast state changes using the tw o component accretion flow model in which the accretion disk consists of a geometrically thin (and optically thick) standard disk coexisting with an a dvection-dominated hot thick disk.