AAS 198th Meeting, June 2001
Session 27. Neutron Stars, Pulsars Gamma ray Bursts
Oral, Monday, June 4, 2001, 2:00-3:30pm, C107

## [27.09] Towards a Theory of Relativistic Collisionless Shocks

S. Nikto (MIPT), M. V. Medvedev (CITA)

It is a customary in astrophysics to describe cosmological shocks, such as in GRBs, Blazar jets, supernovae, in the {\em MHD approximation}. This is often done, without any justification, even for {\em collisionless} shocks in which the particle mean free path exceeds the size of the system. Here we demonstrate that shocks are the sites of strong kinetic (particle) instabilities. These instabilities may naturally produce random electric and magnetic fields which are able to scatter the particles over pitch angle (thermalize). This validates the use of MHD and the shock jump conditions in particular. The effective collisions are also necessary for the efficient Fermi acceleration of particles.

We demonstrate that ultra-relativistic shocks are dominated by random magnetic fields, which turn out to reach the sub-equipartition strength, rather than electric fields, because the relativistic two-stream instability is much more efficient than the beam plasma instability. We also discuss the decay of the produced fields and its effect on the structure of the shocks. We present various observational predictions for GRBs and internal shocks in blazar jets. In particular, we demonstrate that the emitted (jitter) radiation have very distinct spectral and polarization properties which may be easily detected.