Intrinsic physical properties and Doppler boosting effects in LS I +61

Aims. Our aim is to show how variable Doppler boosting of an intrinsically variable jet can explain the long-term modulation of 1667 ± 8 days observed in the radio emission of LS I +61°303. Methods. The physical scenario is that of a conical, magnetized plasma jet having a periodical (P1) increase of relativistic particles, Nrel, at a specific orbital phase, as predicted by accretion in the eccentric orbit of LS I +61°303. Jet precession (P2) changes the angle, η, between jet axis and line of sight, thereby inducing variable Doppler boosting. The problem is defined in spherical geometry, and the optical depth through the precessing jet is calculated by taking into account that the plasma is stratified along the jet axis. The synchrotron emission of such a jet was calculated and we fitted the resulting flux density Smodel(t) to the observed flux density obtained during a 6.5-year monitoring of LS I +61°303 by the Green Bank radio interferometer. Results. Our physical model for the system LS I +61°303 is not only able to reproduce the long-term modulation in the radio emission, but it also reproduces all the other observed characteristics of the radio source, the orbital modulation of the outbursts, their orbital phase shift, and their spectral index properties. Moreover, a correspondence seems to exist between variations in the ejection angle induced by precession and the rapid rotation in position angle observed in VLBA images. Conclusions. The peak of the long-term modulation occurs when the jet electron density is around its maximum and the approaching jet is forming the smallest possible angle with the line of sight. This coincidence of maximum number of emitting particles and maximum Doppler boosting of their emission occurs every ~1667 days and creates the long-term modulation observed in LS I +61°303