Temporal features of LS I +61°303 in hard X-rays from the Swift/BAT survey data

Abstract

We study the long-term spectral and timing behaviour of LS I +61°303 in hard X-rays (15-150 keV) using ∼10 years of survey data from the Swift Burst Alert Telescope (BAT) monitor. We focus on the detection of long periodicities known to be present in this source in multiple wavelengths. We clearly detect three periods: the shorter one at 26.48 d is compatible with the orbital period of the system; the second, longer, periodicity at 26.93 d, is detected for the first time in X-rays and its value is consistent with an analogous temporal feature recently detected in the radio and in the gamma-ray waveband, and we associate it with a modulation caused by a precessing jet in this system. Finally, we find also evidence of the long-term periodicity at ∼1667 d, that results compatible with a beat frequency of the two close, and shorter, periodicities. We discuss our results in the context of the multiband behaviour of the physical processes of this source.