Witnessing the Gradual Slowdown of Powerful Extragalactic Jets: The X-Ray-Optical-Radio Connection

A puzzling feature of the {\it Chandra}--detected quasar jets is that theirX-ray emission decreases faster along the jet than their radio emission,resulting to an outward increasing radio to X-ray ratio. In some sources thisbehavior is so extreme that the radio emission peak is located clearlydownstream of that of the X-rays. This is a rather unanticipated behavior giventhat the inverse Compton nature of the X-rays and the synchrotron radio emission areattributed to roughly the same electrons of the jet's non-thermal electrondistribution. In this note we show that this morphological behavior can resultfrom the gradual deceleration of a relativistic flow and that the offsets inpeak emission at different wavelengths carry the imprint of this deceleration.This notion is consistent with another recent finding, namely that the jetsfeeding the terminal hot spots of powerful radio galaxies and quasars are stillrelativistic with Lorentz factors $\Gamma \sim 2-3$. The picture of thekinematics of powerful jets emerging from these considerations is that theyremain relativistic as they gradually decelerate from Kpc scales to the hotspots, where, in a final collision with the intergalactic medium, theyslow-down rapidly to the subrelativistic velocities of the hot spot advancespeed.Comment: Submitted in ApJ Letters on Jan. 14, 200