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

Abstract

From the longest monitoring of LS I +61°303 done to date by the Rossi X-ray Timing Explorer 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 light curves 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 ± 44.6 day 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 light curves, compatible with the former results, including the shift. © 2012. The American Astronomical Society. All rights reserved.