Fig. 4
From: A 34.5 day quasi-periodic oscillation in γ-ray emission from the blazar PKS 2247–131
Schematic illustration of a helical jet that produces periodically modulated emission. The emitting blob’s motion has a pitch angle ϕ from the jet’s axis, which has an inclination angle ψ from the line of sight. As the emitting blob moves towards the observer, the viewing angle to the blob changes periodically
