LONG-TERM X-RAY MONITORING OF LS I+61°303: ANALYSIS OF SPECTRAL VARIABILITY AND FLARES

We report on the full analysis of a Rossi X-ray Timing Explorer Proportional Counter Array monitoring of the gamma-ray binary system LS I +61 degrees 303. The data set covers 42 contiguous cycles of the system orbital motion. Analyzing this X-ray monitoring data set, the largest to date for this source, we report on the variability of the orbital profile and the spectral distribution, and provide strong evidence for an anti-correlation between flux and spectral index (the higher the flux, the harder the spectral index). Furthermore, we present the analysis of two newly discovered kilosecond-timescale flares, which present significant variability also on shorter timescales and tend to occur at orbital phases between 0.6 and 0.9. However, a detailed timing analysis of the flares does not show any coherent or quasi-coherent (QPO) structure in their power spectra. We also investigated the possible appearance of the radio super-orbital modulation at X-ray energies, but we could not unambiguously detect such modulation in the system flux history nor in the evolution of its orbital modulation fraction.