Relativistic Jet Response to Precession and Wave-Wave Interactions

Three dimensional numerical simulations of the response of a Lorentz factor 2.5 relativistic jet to precession at three different frequencies have been performed. Low, moderate and high precession frequencies have been chosen relative to the maximally unstable frequency predicted by a Kelvin-Helmholtz stability analysis. Transverse motion and velocity decreases as the precession frequency increases. Although helical displacement of the jet decreases in amplitude as the precession frequency increases, a helical shock is generated in the medium external to the jet at all precession frequencies. Complex pressure and velocity structure inside the jet is shown to be produced by a combination of the helical surface and first body modes predicted by a normal mode analysis of the relativistic hydrodynamic equations. The surface and first body mode have different wave speed and wavelength, are launched in phase by the periodic precession, and exhibit beat patterns in synthetic emission images. Wave (pattern) speeds range from $0.41c$ to $0.86c$ but beat patterns remain stationary. Thus, we find a mechanism that can produce differentially moving and stationary features in the jet.