Jet stability and the generation of superluminal and stationary components

We present a numerical simulation of the response of an expanding relativis tic jet to the ejection of a superluminal component. The simulation has bee n performed with a relativistic time-dependent hydrodynamical code from whi ch simulated radio maps are computed by integrating the transfer equations for synchrotron radiation. The interaction of the superluminal component wi th the underlying jet results in the formation of multiple conical shocks b ehind the main perturbation. These trailing components can be easily distin guished because they appear to be released from the primary superluminal co mponent instead of being ejected from the core. Their oblique nature should also result in distinct polarization properties. Those appearing closer to the core show small apparent motions and a very slow secular decrease in b rightness and could be identified as stationary components. Those appearing farther downstream are weaker and can reach superluminal apparent motions. The existence of these trailing components indicates that not all observed components necessarily represent major perturbations at the jet inlet; rat her, multiple emission components can be generated by a single disturbance in the jet. While the superluminal component associated with the primary pe rturbation exhibits a rather stable pattern speed, trailing components have velocities that increase with distance from the core but move at less than the jet speed. The trailing components exhibit motion and structure consis tent with the triggering of pinch modes by the superluminal component. The increase in velocity of the trailing components is an indirect consequence of the acceleration of the expanding fluid, which is assumed to be relativi stically hot; if observed, such accelerations would therefore favor an elec tron-positron (as opposed to proton rest mass) dominated jet.