Owens Valley Radio Observatory monitoring of LS I +61°303 completes three cycles of the super-orbital modulation

Abstract

The high-mass X-ray binary LS I +61°303 is composed of a Be-type star and a compact object. The emission is variable and periodic across the electromagnetic spectrum, from radio to very high-energy gamma rays. The orbital period is ~26.5 d, and the source also features a super-orbital period with a value of ~4.6 years. Long-term monitoring of the binary by the Owens Valley Radio Observatory (OVRO) at 15 GHz has now completed 13.8 years, which corresponds to three full cycles of the super-orbital period. We performed a timing analysis of the OVRO radio light curve, and we also combined the OVRO data with the full archive of previous radio observations and computed the discrete autocorrelation function. The most powerful features in the periodogram of the OVRO data are two peaks at P1 = 26.49 +/- 0.05 d and P2 = 26.93 +/- 0.05 d. Our measurement of the long-term period is P_long = 1698 +/- 196 d. Dividing the OVRO data into three segments of equal length showed that the two periods, P1 and P2, are present in the periodogram of each of the consecutive long-term cycles. Our analysis of the full radio archive resulted in the detection of the same three periods, and the autocorrelation function showed a regular pattern, proving the stability of the super-orbital modulation. We report a possible systematic modulation of the radio flux density with a timescale of approximately 40 years that has so far remained unnoticed. The physical model of a relativistic jet whose mass loading is modulated with the orbital period P1 and is precessing with the slightly larger period P2, giving rise to a beating with period P_long, had previously been able to reproduce the radio and gigaelectron volt emission. The ongoing presence and the stability of the periodic signals imply that this model is still the most plausible explanation for the physical processes at work in this source. (abridged)

12 pages, 8 figures, 4 appendices (3 additional figures and 2 tables), accepted for publication in A&A

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