Particle Acceleration in Relativistic Jets Due to Weibel Instability

Shock acceleration is an ubiquitous phenomenon in astrophysical plasmas.Plasma waves and their associated instabilities (e.g., the Buneman instability,two-streaming instability, and the Weibel instability) created in the shocksare responsible for particle (electron, positron, and ion) acceleration. Usinga 3-D relativistic electromagnetic particle (REMP) code, we have investigatedparticle acceleration associated with a relativistic jet front propagatingthrough an ambient plasma with and without initial magnetic fields. We findonly small differences in the results between no ambient and weak ambientmagnetic fields. Simulations show that the Weibel instability created in thecollisionless shock front accelerates particles perpendicular and parallel tothe jet propagation direction. While some Fermi acceleration may occur at thejet front, the majority of electron acceleration takes place behind the jetfront and cannot be characterized as Fermi acceleration. The simulation resultsshow that this instability is responsible for generating and amplifying highlynonuniform, small-scale magnetic fields, which contribute to the electron'stransverse deflection behind the jet head. The ``jitter'' radiation (Medvedev2000) from deflected electrons has different properties than synchrotronradiation which is calculated in a uniform magnetic field. This jitterradiation may be important to understanding the complex time evolution and/orspectral structure in gamma-ray bursts, relativistic jets, and supernovaremnants.