  Using our new 3-D relativistic electromagnetic particle (REMP) code
parallelized with MPI, we investigated long-term particle acceleration
associated with a relativistic electron-positron jet propagating in an
unmagnetized ambient electron-positron plasma. The simulations were performed
using a much longer simulation system than our previous simulations in order to
investigate the full nonlinear stage of the Weibel instability and its particle
acceleration mechanism. Cold jet electrons are thermalized and ambient
electrons are accelerated in the resulting shocks. Acceleration of ambient
electrons leads to a maximum ambient electron density three times larger than
the original value. Behind the bow shock in the jet shock strong
electromagnetic fields are generated. These fields may lead to time dependent
afterglow emission. We calculated radiation from electrons propagating in a
uniform parallel magnetic field to verify the technique. We also used the new
technique to calculate emission from electrons based on simulations with a
small system. We obtained spectra which are consistent with those generated
from electrons propagating in turbulent magnetic fields with red noise. This
turbulent magnetic field is similar to the magnetic field generated at an early
nonlinear stage of the Weibel instability. A fully developed shock within a
larger system generates a jitter/synchrotron spectrum.
