Acceleration of UHE Cosmic Ray Particles at Relativistic Jets in Extragalactic Radio Sources

A mechanism of ultra-high energy cosmic ray acceleration in extragalactic radio sources, at the interface between the relativistic jet and the surrounding medium, is discussed as a supplement to the shock acceleration in `hot spots'. Due to crossing the tangential discontinuity of the velocity the particle can gain an amount of energy comparable to the energy gain at the shock crossing. However, the spectrum of particles accelerated at the jet side boundary is expected to be much flatter than the one formed at the shock. Due to this fact, particles accelerated at the boundary can dominate the overall spectrum at highest energies. In conditions characteristic to extragalactic jets' terminal shocks, the mechanism naturally provides the particles with E ~ 10^20 eV and complies with the efficiency requirements. The spectrum formation near the cut-off energy due to action of both the shock acceleration and the tangential discontinuity acceleration is modelled with the Monte Carlo particle simulations. It confirms that the upper energy limit can surpass the shock acceleration estimate.