Volume 585, January 2016
|Number of page(s)||6|
|Published online||07 January 2016|
Origin of the long-term modulation of radio emission of LS I +61°303
1 Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
2 INAF – Osservatorio Astrofisico di Arcetri, L.go E. Fermi 5, Firenze, Italy
Received: 10 July 2015
Accepted: 26 October 2015
Context. One of the most unusual aspects of the X-ray binary LS I +61°303 is that at each orbit (P1 = 26.4960 ± 0.0028 d) one radio outburst occurs whose amplitude is modulated with Plong, a long-term period of more than 4 yr. It is still not clear whether the compact object of the system or the companion Be star is responsible for the long-term modulation.
Aims. We study here the stability of Plong. Such a stability is expected if Plong 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 Plong is related to variations in the mass loss of the companion Be star.
Methods. We built a database of 36.8 yr 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.
Results. In addition to the two dominant features at P1 and P2, the timing analysis gives a feature at Plong = 1628 ± 48 days. The determined value of Plong agrees with the beat of the two dominant features, i.e. Pbeat = 1 / (ν1−ν2) = 1626 ± 68 d. Lomb-Scargle results of radio data and model data compare very well. The correlation coefficient of the radio data oscillates at multiples of Pbeat, as does the correlation coefficient of the model data.
Conclusions. 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 and creates the long-term modulation observed in LS I +61°303.
Key words: stars: emission-line, Be / gamma rays: stars / X-rays: individuals: LS I +61303 / radio continuum: stars / stars: jets / X-rays: binaries
© ESO, 2016
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