Geometry of the precession model (not to scale) adopted from Qian (2011). The Z-axis, which is taken to be the axis of precession, is defined by parameters (ϵ, ψ); the Yp axis indicates the direction toward the observer; the position of a knot (superluminal component) is defined by parameters (A,Φ); A(Z) is the amplitude function and Φ is the azimuthal angle (or phase); the plane (Xp,Zp) represents the plane of the sky; and θ denotes the angle between the knot’s velocity vector (S) and the direction toward the observer (V). η is the initial half opening angle of the precessing jet cone (see text).
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