Electron‐Ion Coupling Upstream of Relativistic Collisionless Shocks

It is argued and demonstrated by particle-in-cell simulations that thesynchrotron maser instability could develop at the front of a relativistic,magnetized shock. The instability generates strong low-frequencyelectromagnetic waves propagating both upstream and downstream of the shock.Upstream of the shock, these waves make electrons lag behind ions so that alongitudinal electric field arises and the electrons are accelerated up to theion kinetic energy. Then thermalization at the shock front results in a plasmawith equal temperatures of electrons and ions. Downstream of the shock, theamplitude of the maser-generated wave may exceed the strength of theshock-compressed background magnetic field. In this case the shock-acceleratedparticles radiate via nonlinear Compton scattering rather than via asynchrotron mechanism. The spectrum of the radiation differs, in thelow-frequency band, from that of the synchrotron radiation, providing possibleobservational tests of the model.Comment: 22 pages, 10 figures. To appear in ApJ vol. 65