The efficiency of coherent radiation from relativistic shocks

Abstract

We discuss a mechanism for intense electromagnetic wave emission at an astrophysical relativistic shock in a magnetized collisionless plasma. At the magnetized shock, the particle reflection by a compressed magnetic field of the shock produces a ring-like distribution in momentum, which gives rise to plasma instabilities. Intense and coherent high-frequency electromagnetic waves will be emitted if the synchrotron maser instability (SMI) is excited, whereas non-propagating magnetic fluctuations will be generated when the Weibel instability (WI) is the dominant mode. The problem is of great astrophysical interest because if intense radiation is emitted, the interaction with the upstream medium induces a large-amplitude electrostatic field (or Wakefield), which may play a role for the acceleration of ultra-high-energy cosmic rays. We review our recent effort to measure the efficiency of the electromagnetic wave emission using fully self-consistent, two-dimensional (2D) particle-in-cell (PIC) simulations for pair plasmas. We found that the emission efficiency in 2D was systematically lower than one dimensional (1D) PIC simulation results. However, the power remains finite even when the WI is active to generate large-amplitude magnetic fluctuations. Astrophysical implications of the present results are briefly discussed.