Inner accretion disk disappearance during a radio flare in GRS 1915+105

Simultaneous X-ray (BeppoSAX, 0.1-300 keV) and radio (Ryle Telescope, 15 GHz) observations of the superluminal galactic source GRS 1915+105 in high flux state were carried out starting October 1996. The source was in a phase of large X-ray intensity and spectral variability. Among the simultaneous observations, an isolated radio flare lasting similar to 2000 s was detected accompanied by X-ray outbursting activity. Here we present the simultaneous radio/X-ray light curves of the flare event and the spectral and timing analysis of the relevant BeppoSAX data. The broad band (0.1-200 keV) energy spectrum of the source can be described in terms of the 'standard' spectral model for black hole candidates. We obtain evidence for a temporary disappearance and subsequent restoring of the inner portion of the accretion disk during the flare from the spectral analysis of the X-ray data. We find a variable power law photon index positively correlated to the variability of the exponential cut-off energy. In addition, frequency and intensity variations of a similar to 4-5 Hz quasi-periodic oscillation (QPO) are detected during the same time interval. The centroid frequency of the QPO is found to be correlated to the spectral power law photon index, and to the X-ray flux from the disk and the high energy flux from the power law. These observational results are discussed in the framework of the disk-instability model proposed for this source.