  Our aim is to show how variable Doppler boosting of an intrinsically variable
jet can explain the long-term modulation of 1667 \pm 8 days observed in the
radio emission of LSI+61303. 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 LSI+61303. Jet precession (P2) changes the angle, eta,
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 LSI+61303 by the Green
Bank radio interferometer. Our physical model for the system LSI+61303 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. We conclude that 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 LSI+61303.
