Particle Acceleration by Pickup Process Upstream of Relativistic Shocks

Particle acceleration at magnetized purely perpendicular relativistic shocks in electron-ion plasmas is studied by means of two-dimensional particle-in-cell simulations. Magnetized shocks with the upstream bulk Lorentz factor gamma(1) >> 1 are known to emit intense electromagnetic waves from the shock front, which induce electrostatic plasma waves (wakefield) and transverse filamentary structures in the upstream region via stimulated/induced Raman scattering and filamentation instability, respectively. The wakefield and filaments inject a fraction of the incoming particles into a particle acceleration process, in which particles are once decoupled from the upstream bulk flow by the wakefield, and are picked up again by the flow. The picked-up particles are accelerated by the motional electric field. The maximum attainable Lorentz factor is estimated as gamma(max,e) similar to alpha gamma(3)(1) for electrons and gamma(max,i) similar to (1 + m(e) gamma(1)/(m)(i))gamma(2)(1) for ions, where alpha similar to 10 is determined from our simulation results. alpha can increase up to gamma(1) for a weakly magnetized shock if gamma(1) is sufficiently large. This result indicates that highly relativistic astrophysical shocks such as external shocks of gamma-ray bursts can be an efficient particle accelerator.