Paper detail

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

Context: In the binary system LSI+61303 the peak flux density of the radio outburst, which is related to the orbital period of 26.4960 +/- 0.0028d, exibits a modulation of 1667 +/- 8 d. The radio emission at high spatial resolution appears structured in a precessing jet with a precessional period of 27-28 d. Aims: How close is the precessional period of the radio jet to the orbital period? Any periodicity in the radio emission should be revealed by timing analysis. The aim of this work is to establish the accurate value of the precessional period. Methods: We analyzed 6.7 years of the Green Bank Interferometer database at 2.2 GHz and 8.3 GHz with the Lomb-Scargle and phase dispersion minimization (PDM) methods and performed simulations. Results: The periodograms show two periodicities, P1 = 26.49 +/- 0.07 d (\nu1=0.03775 d^{-1}) and P2 = 26.92 +/- 0.07 d (\nu2 = 0.03715 d^{-1}). Whereas radio outbursts have been known to have nearly orbital occurrence P1 with timing residuals exhibiting a puzzling sawtooth pattern, we probe in this paper that they are actually periodical outbursts and that their period is Paverage= (2/(\nu1 + \nu2)= 26.70 +/- 0.05 d. The period Paverage as well as the long-term modulation Pbeat=1/(\nu1 - \nu2)=1667 +/- 393 d result from the beat of the two close periods, the orbital P1 and the precessional P2 periods. Conclusions: The precessional period, indicated by the astrometry to be of 27--28 d, is P2=26.92 d. The system \lsi seems to be one more case in astronomy of beat, i.e., a phenomenon occurring when two physical processes create stable variations of nearly equal frequencies. The very small difference in frequency creates a long-term variation of period 1/(\nu1-\nu2). The long-term modulation of 1667 d results from the beat of the two close orbital and precessional rates.