Ultra-high energy cosmic rays from shocks in the lobes of powerful radio
  galaxies
  The origin of ultra-high energy cosmic rays (UHECRs) has been an open
question for decades. Here, we use a combination of hydrodynamic simulations
and general physical arguments to demonstrate that UHECRs can in principle be
produced by diffusive shock acceleration (DSA) in shocks in the backflowing
material of radio galaxy lobes. These shocks occur after the jet material has
passed through the relativistic termination shock. Recently, several authors
have demonstrated that highly relativistic shocks are not effective in
accelerating UHECRs. The shocks in our proposed model have a range of
non-relativistic or mildly relativistic shock velocities more conducive to
UHECR acceleration, with shock sizes in the range 1-10kpc. Approximately 10% of
the jet's energy flux is focused through a shock in the backflow of $M>3$.
Although the shock velocities can be low enough that acceleration to high
energy via DSA is still efficient, they are also high enough for the Hillas
energy to approach $10^{19-20}$eV, particularly for heavier CR composition and
in cases where fluid elements pass through multiple shocks. We discuss some of
the more general considerations for acceleration of particles to ultra-high
energy with reference to giant-lobed radio galaxies such as Centaurus A and
Fornax A, a class of sources which may be responsible for the observed
anisotropies from UHECR observatories.
