2016A&A...585A.123M

One of the most unusual aspects of the X-ray binary LS I +61°303 is that at each orbit (P₁=26.4960±0.0028d) one radio outburst occurs whose amplitude is modulated with P_long, a long-term period of more than 4yr. It is still not clear whether the compact object of the system or the companion Be star is responsible for the long-term modulation. We study here the stability of P_long. Such a stability is expected if P_long is due to periodic (P₂) Doppler boosting of periodic (P₁) ejections from the accreting compact object of the system. On the contrary it is not expected if P_long is related to variations in the mass loss of the companion Be star. We built a database of 36.8yr of radio observations of LS I +61°303 covering more than 8 long-term cycles. We performed timing and correlation analysis. We also compared the results of the analyses with the theoretical predictions for a synchrotron emitting precessing (P₂) jet periodically (P₁) refilled with relativistic electrons. In addition to the two dominant features at P₁ and P₂, the timing analysis gives a feature at P_long=1628 ±48 days. The determined value of P_long agrees with the beat of the two dominant features, i.e. P_beat=1/(ν₁-ν₂)=1626±68d. Lomb-Scargle results of radio data and model data compare very well. The correlation coefficient of the radio data oscillates at multiples of P_beat, as does the correlation coefficient of the model data. Cycles in varying Be stars change in length and disappear after 2-3 cycles following the well-studied case of the binary system ζ Tau. On the contrary, in LS I +61°303 the long-term period is quite stable and repeats itself over the available 8 cycles. The long-term modulation in LS I +61°303 accurately reflects the beat of periodical Doppler boosting (induced by precession) with the periodicity of the ejecta. The peak of the long-term modulation occurs at the coincidence of the maximum number of ejected particles with the maximum Doppler boosting of their emission; this coincidence occurs every (1)/(ν₁-ν₂) and creates the long-term modulation observed in LS I +61°303.