Unveiling the super-orbital modulation of LS I +61 303 in X-rays

TL;DR

This study presents the first evidence of a long-term super-orbital modulation in X-ray emission from LS I +61 303, revealing a 1667-day cycle and a phase shift relative to radio observations, based on extensive RXTE data.

Contribution

It is the first to identify super-orbital modulation in X-ray emission of LS I +61 303 and compares it with radio data, revealing a phase shift.

Findings

Detected a 1667-day super-orbital period in X-ray emission.

Found a phase shift of approximately 282 days between X-ray and radio super-orbital modulations.

Confirmed super-orbital modulation in maximum count rate of orbital lightcurves.

Abstract

From the longest monitoring of LS I +61 303 done to date by the Rossi X-ray Timing Explorer (RXTE) we found evidence for the long-sought, years- long modulation in the X-ray emission of the source. The time evolution of the modulated fraction in the orbital lightcurves can be well fitted with a sinusoidal function having a super-orbital period of 1667 days, the same as the one reported in non-contemporaneous radio measurements. However, we have found a 281.8 \pm 44.6 days shift between the super-orbital variability found at radio frequencies extrapolated to the observation time of our campaign and what we found in the super-orbital modulation of the modulated fraction of our X-ray data. We also find a super-orbital modulation in the maximum count rate of the orbital lightcurves, compatible with the former results, including the shift.