  Observations of gamma-ray-bursts and jets from active galactic nuclei reveal
that the jet flow is characterized by a high radiative efficiency and that the
dissipative mechanism must be a powerful accelerator of non-thermal particles.
Shocks and magnetic reconnection have long been considered as possible
candidates for powering the jet emission. Recent progress via fully-kinetic
particle-in-cell simulations allows us to revisit this issue on firm physical
grounds. We show that shock models are unlikely to account for the jet
emission. In fact, when shocks are efficient at dissipating energy, they
typically do not accelerate particles far beyond the thermal energy, and vice
versa. In contrast, we show that magnetic reconnection can deposit more than
50% of the dissipated energy into non-thermal leptons as long as the energy
density of the magnetic field in the bulk flow is larger than the rest mass
energy density. The emitting region, i.e., the reconnection downstream, is
characterized by a rough energy equipartition between magnetic fields and
radiating particles, which naturally accounts for a commonly observed property
of blazar jets.
