Temporal features of LS I +61$^{\circ}$303 in hard X-rays from the Swift/BAT survey data

TL;DR

This study analyzes over a decade of Swift/BAT data to identify and characterize multiple long-term periodicities in the hard X-ray emission of LS I +61$^{ullet}$303, revealing new insights into its orbital and jet precession behaviors.

Contribution

First detection of the 26.93-day precession period in X-rays, linking it to radio and gamma-ray observations, and identification of a long-term 1667-day cycle in hard X-ray data.

Findings

Detection of the orbital period at 26.48 days.

First X-ray detection of the 26.93-day precession period.

Evidence of a 1667-day long-term periodicity.

Abstract

We study the long-term spectral and timing behaviour of LS I +61$^{\circ}$303 in hard X-rays (15--150 keV) using $\sim$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 days is compatible with the orbital period of the system; the second, longer, periodicity at 26.93 days, 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 $\sim$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 multi-band behaviour of the physical processes of this source.