Rossi x-ray timing explorer observations of GRS 1915+105

The Galactic superluminal motion source GRS 1915+105 was observed with the Rossi X-Ray Timing Explorer satellite at several occasions during its ongoing active state. The observed X-ray intensity changes drastically on a variety of timescales ranging from subseconds to days. In particular, the source exhibits quasi-periodic brightness sputters with varying duration and repetition timescale. These episodes occur occasionally, while the more common X-ray intensity variations are faster with much smaller amplitudes. The spectrum during the brightness sputters is remarkably different from the spectrum of the mean high-state emission. We argue that such sputtering episodes are possibly caused by a major accretion disk instability. Based on the coincidence in time of two radio flares following the observed X-ray sputtering episodes, we speculate that superluminal ejections (as observed from GRS 1915+105 during earlier activity periods) are related to episodes of large-amplitude X-ray variations.