  We summarize latest PIC simulation results on the radiation from Poynting
jets and strongly magnetized collisionless shocks when a Poynting jet runs into
cold ambient medium. We find that in all cases the radiative power output is
much below that predicted by synchrotron radiation and the critical frequency
is also much lower than the synchrotron critical frequency. We discuss the
implications for the interpretation of GRB prompt emission data.
