2 Years of INTEGRAL Monitoring of GRS 1915+105. II. X-Ray Spectro-Temporal Analysis

This is the second paper presenting the results of 2 yr of monitoring of GRS 1915+105 with INTEGRAL, RXTE, and the Ryle Telescope. We present the X-ray spectral and temporal analysis of four observations showing strong radio to X-ray correlations. During one observation GRS 1915+105 was in a steady state, while during the three others it showed cycles of X-ray dips and spikes (followed by radio flares). Through time-resolved spectroscopy of these cycles, we suggest that the soft X-ray spike is the trigger of the ejection. The ejected medium is then the coronal material responsible for the hard X-ray emission. In the steady state observation, the X-ray spectrum is indicative of the hard-intermediate state, with the presence of a relatively strong emission at 15 GHz. The X-ray spectrum is the sum of a Comptonized component and an extra power law extending to energies >200 keV without any evidence for a cutoff. We observe a possible correlation of the radio flux with that of the power law component, which may indicate that we see direct emission from the jet at hard X-ray energies. We study the energy dependence of a ~4 Hz QPO during the hard-intermediate state observation. The QPO ``spectrum'' is well modeled by a power law with a cutoff at an energy of about 11 keV, and clearly differs from the relative contribution of the Comptonized component to the overall flux. This may rule out models of global oscillations of the Compton corona.