On possible `cosmic ray cocoons' of relativistic jets

Abstract

We consider effects on an (ultra-) relativistic jet and its ambient medium caused by high energy cosmic rays accelerated at the jet side boundary. As illustrated by simple models, during the acceleration process a flat cosmic ray distribution can be created, with gyroradia for highest particles' energies reaching the scales comparable to the jet radius or the energy density comparable to the ambient medium pressure. In the case of efficient radiative losses a high energy bump in the spectrum can dominate the cosmic ray pressure. In extreme cases the cosmic rays are able to push the ambient medium off, providing a `cosmic ray cocoon' separating the jet from the surrounding medium. The considered cosmic rays provide an additional jet breaking force and lead to a number of consequences for the jet structure and its radiative output. In particular the involved dynamic and acceleration time scales are in the range observed in variable AGNs.