  What are the mechanisms of particle acceleration and radiation, as well as
magnetic field build up and decay in relativistic shocks are open questions
with important implications to various phenomena in high energy astrophysics.
While the Weibel instability is possibly responsible for magnetic field build
up and diffusive shock acceleration is a model for acceleration, both have
problems and current PIC simulation show that particles are accelerated only
under special conditions and the magnetic field decays on a short length scale.
We present here a novel model for the structure and the emission of highly
relativistic collisionless shocks. The model takes into account (and is based
on) non-local energy and momentum transport across the shock front via emission
and absorption of high-energy photons. This leads to a pre-acceleration of the
fluid and pre-amplificaiton of the magnetic fields in the upstream region. Both
have drastic implications on the shock structure. The model explains the
persistence of the shock generated magnetic field at large distances from the
shock front. The dissipation of this magnetic field results in a continuous
particle acceleration within the downstream region. The model suggests two
non-uniform emission zones (the downstream and the upstream), that give rise to
three emission components with different spectral and temporal properties. A
unique feature of the model is the existence of an "attractor", toward which
any shock will evolve. This enables us to estimate from first principles the
synchrotron and inverse Compton spectrum of the downstream emission. The model
is applicable to any relativistic shock, but its distinctive features show up
only for large compactness. We demonstrate that prompt and afterglow Gamma-Ray
Bursts' shocks satisfy the relevant conditions and compare their observations
with the predictions.
