  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. We have also performed
simulations with electron-ion jets. 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 for electron-positron jets
and its particle acceleration mechanism. Cold jet electrons are thermalized and
ambient electrons are accelerated in the resulting shocks for both cases.
Acceleration of ambient electrons leads to a maximum ambient electron density
three times larger than the original value for pair plasmas. 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 with two different cases for Lorentz
factors (15 and 100). 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.
