  It has been suggested that relativistic blast waves may power the jets of AGN
and gamma-ray bursts (GRB). We address the important issue how the kinetic
energy of collimated blast waves is converted into radiation. It is shown that
swept-up ambient matter is quickly isotropised in the blast wave frame by a
relativistic two-stream instability, which provides relativistic particles in
the jet without invoking any acceleration process. The fate of the blast wave
and the spectral evolution of the emission of the energetic particles is
therefore solely determined by the initial conditions. We compare our model
with existing multiwavelength data of AGN and find remarkable agreement.
