Particle acceleration at shock fronts in AGNs and jets

Abstract

Several aspects of the theory of first order Fermi acceleration at shock fronts are discussed which may be relevant for active galactic nuclei (AGNs) and jets. In the test particle picture, recent work on acceleration at oblique shocks is reviewed which indicates that such shocks produce a harder spectrum than parallel shocks. They also tend to accumulate a substantial excess of energetic particles ahead of the front (snow plough effect. Next, models predicting the maximum energy to which electrons can be accelerated at hot spots in jets are discussed. These models are successful in explaining the observed synchrotron spectrum, but operate only if the Alfvén turbulence on each side of the shock front is of a very special nature. Finally, some recent work on the nonlinear problem of the back reaction of accelerated particles on a relativistic fluid flow is summarized, which indicates that higher acceleration efficiencies may be achieved in relativistic shocks than can be obtained in nonrelativistic shocks.