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

Abstract

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.

arXiv comment8 pages, 4 figures, accepted for publication in A&A

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• Study of orbital and superorbital variability of LSI +61 303 with X-ray data
• Orbital variability of the optical linear polarization of the $\gamma$-ray binary LS I +61 303 and new constraints on the orbital parameters
• Spectral analysis in orbital/superorbital phase space and hints of superorbital variability in the hard X-rays of LS I +61 303
• Discovery of a periodical apoastron GeV peak in LS I +61°303
• Long-term OVRO monitoring of LSI+61303: confirmation of the two close periodicities
• The broad-band radio spectrum of LSI+61303 in outburst
• Evidence for periodic accretion-ejection in LSI+61303
• Connection between orbital modulation of H-alpha and gamma-rays in the Be/X-ray binary LSI+61303
• The Multi-wavelength Characteristics of the TeV Binary LS I +61${}^{\circ }$ 303
• Understanding the periodicities in radio and GeV emission from LS I+61303
• Simultaneous long-term monitoring of LS I +61°303 by OVRO and Fermi-LAT
• Improved binary solution for the gamma-ray binary 1FGL J1018.6-5856
• A Precessing Jet Scenario for the Multi-Wavelength Long-Term Modulation of LS I +61${}^{\circ }$303
• Temporal features of LS I +61${}^{\circ }$303 in hard X-rays from the Swift/BAT survey data
• Radius, rotational period and inclination of the Be stars in the Be/gamma-ray binaries MWC 148 and MWC 656
• Owens Valley Radio Observatory monitoring of LS I +61°303 completes three cycles of the super-orbital modulation
• Rapid X-ray variability of the gamma-ray binary LS I+61 303
• VERITAS observations of exceptionally bright TeV flares from LS I +61${}^{\circ }$ 303