  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 $\nu
\propto t^{-3/2}$.
