Numerical Simulations of a Restarting Jet

The results from a numerical simulation of an intermittent jet are presented to address the applicability of a "restarting jet" model to the one-sided jets prevalent in the Fanaroff & Riley Class II radio sources. Intermittently restarting a jet requires that new jets be launched into lobes containing thermalized material from previous jet activity. This scenario is tested numerically, and several properties of a restarting jet which hitherto have been unappreciated are discussed and compared with known trends in classical doubles. It is found that a restarted jet is "overdense" relative to the old jet material through which it passes. The advance speed of the working surface of the restarted jet is greater than that of the original jet, while the advance speed Mach number of the restarted jet is less than that of the original jet. There are two distinct phases of a "partial jet" in which the jet is not continuous between the core and the maximum extent of the lobe. The combined lifetimes of the partial jets expressed as a fraction of the time between successive jet launchings (the "duty cycle") approaches unity as the number of elapsed duty cycles since creation of the radio source increases. A bow shock of comparable strength to the terminal Mach disc in the restarted jet is excited in the old jet material and it is predicted that if a radio source has a prominent partial jet, then there should be a prominent feature leading the partial jet associated with the bow shock. Finally, the hot spot in the radio lobe quickly dissipates once the momentum flux of the jet has been terminated.