Relativistic beaming effects due to the travel along a helical path within the jet. Top panel: Flux variability amplitude caused by the evolution of the Doppler factor, δ, when an emitting zone is traveling downstream along a helical path. The variability amplitude is plotted as a function of the phase of the spiral rotation. The horizontal dotted blue line gives the observed variability amplitude in the 3–7 keV band. Bottom panel: Corresponding Doppler factor, δ, as a function of the spiral rotation phase. The curves are produced for different jet axis angles (θ) relative to the line of sight. The zone is assumed to move with a Lorentz factor Γ_b = 20 in a helical field with a pitch angle of 2.9° (see the main text for more details).