Top: angle η between jet axis and line of sight at different phases of the long-term modulation (Θ) vs. orbital phase (Φ). One sees as the minimum of η, which corresponds to maximum Doppler boosting, moves at different Φs for the different Θ phases. The black thick curve is (on an arbitrary scale) the adopted relativistic electron density function of P₁ of Sect. 2.4. Bottom: amplitude modulation and orbital shift of the outbursts: outbursts at different Θs vs. Φ (with the same colour code as the above panel). The outburst decreases its amplitude and shifts in orbital phase.