Fast Afterglows of Fast Radio Bursts

The main FRB event may leave behind a clump of relativistic plasma with high ``free energy'' density. As the plasma undergoes collisionless relaxation, it emits coherent electromagnetic waves. These electromagnetic waves may be observable as a fast radio afterglow, with decreasing frequency and intensity. We demonstrate the fast coherent afterglow in a numerical experiment. We tentatively predict the peak afterglow frequency decreasing with time as $\nu \propto t^{-3/2}$.