A puzzling feature of the Chandra –detected quasar jets is that their X-ray emission decreases faster along the jet than their radio emission , resulting to an outward increasing radio to X-ray ratio .
In some sources this behavior is so extreme that the radio emission peak is located clearly downstream of that of the X-rays .
This is a rather unanticipated behavior given that the inverse Compton nature of the X-rays and the synchrotron radio emission are attributed to roughly the same electrons of the jet ’ s non-thermal electron distribution .
In this note we show that this morphological behavior can result from the gradual deceleration of a relativistic flow and that the offsets in peak emission at different wavelengths carry the imprint of this deceleration .
This notion is consistent with another recent finding , namely that the jets feeding the terminal hot spots of powerful radio galaxies and quasars are still relativistic with Lorentz factors \Gamma \sim 2 - 3 .
The picture of the kinematics of powerful jets emerging from these considerations is that they remain relativistic as they gradually decelerate from Kpc scales to the hot spots , where , in a final collision with the intergalactic medium , they slow-down rapidly to the subrelativistic velocities of the hot spot advance speed .