Collimated synchrotron threads in wide-angle-tail radio galaxies: cosmic thunderbolts?

Abstract

The WAT (wide-angle-tail) radio galaxies ESO 137-006 and 3C338 show radio filaments including some extending between the two radio tails. If, in analogy to the Parker solar wind model, jets carry a powerful relativistic electric current, the continuity equation shows that slow return current responding to a temporal variation in jet power would inevitably lead to a localized short-lived charging-up, i.e. a temporary patch of electric field. Progenitors of the observed radio filaments are the relic jets left by repeated episodes of nuclear activity and pushed sideways by the ram pressure, though remaining connected to the broad radio lobes. The Debye shielding distance, for energetic particles with a p−2-distribution of momentum p, allows a very large Debye scale at the highest energies, approaching the radio galaxy scale, with a huge total energy content. Here, we propose that discharges dissipating the transient patches of electric fields through the filaments illuminate them. Our proposal is based on gradient drift currents within the bipolar jets, allowing for drift speeds approaching the speed of light. We sketch a scenario how a discharge current sometimes gets manifested as a radio filament connecting opposite lobes/tails of a WAT radio galaxy.