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Context: In the binary system LSI+61303 the peak flux density of the radiooutburst, which is related to the orbital period of 26.4960 +/- 0.0028d,exibits a modulation of 1667 +/- 8 d. The radio emission at high spatialresolution appears structured in a precessing jet with a precessional period of27-28 d. Aims: How close is the precessional period of the radio jet to theorbital period? Any periodicity in the radio emission should be revealed bytiming analysis. The aim of this work is to establish the accurate value of theprecessional period. Methods: We analyzed 6.7 years of the Green BankInterferometer database at 2.2 GHz and 8.3 GHz with the Lomb-Scargle and phasedispersion minimization (PDM) methods and performed simulations. Results: Theperiodograms show two periodicities, P1 = 26.49 +/- 0.07 d (\nu1=0.03775d^{-1}) and P2 = 26.92 +/- 0.07 d (\nu2 = 0.03715 d^{-1}). Whereas radiooutbursts have been known to have nearly orbital occurrence P1 with timingresiduals exhibiting a puzzling sawtooth pattern, we probe in this paper thatthey are actually periodical outbursts and that their period is Paverage=(2/(\nu1 + \nu2)= 26.70 +/- 0.05 d. The period Paverage as well as thelong-term modulation Pbeat=1/(\nu1 - \nu2)=1667 +/- 393 d result from the beatof the two close periods, the orbital P1 and the precessional P2 periods.Conclusions: The precessional period, indicated by the astrometry to be of27--28 d, is P2=26.92 d. The system \lsi seems to be one more case in astronomyof beat, i.e., a phenomenon occurring when two physical processes create stablevariations of nearly equal frequencies. The very small difference in frequencycreates a long-term variation of period 1/(\nu1-\nu2). The long-term modulationof 1667 d results from the beat of the two close orbital and precessionalrates.

Long-term periodicity in LSI+61303 as beat frequency between orbital and precessional rate