  lasma instabilities excited in collisionless shocks are responsible for
particle acceleration. We have investigated the particle acceleration and shock
structure associated with an unmagnetized relativistic electron-positron jet
propagating into an unmagnetized electron-positron plasma. Cold jet electrons
are thermalized and slowed while the ambient electrons are swept up to create a
partially developed hydrodynamic-like shock structure. In the leading shock,
electron density increases by a factor of about 3.5 in the simulation frame.
Strong electromagnetic fields are generated in the trailing shock and provide
an emission site. These magnetic fields contribute to the electron's transverse
deflection behind the shock. Our initial results of a jet-ambient interaction
with anti-parallel magnetic fields show pile-up of magnetic fields at the
colliding shock, which may lead to reconnection and associated particle
acceleration. We will investigate the radiation in transient stage as a
possible generation mechanism of precursors of prompt emission. In our
simulations we calculate the radiation from electrons in the shock region. The
detailed properties of this radiation are important for understanding the
complex time evolution and spectral structure in gamma-ray bursts, relativistic
jets, and supernova remnants.
