Dissipation of jet bulk kinetic energy in powerful blazars

Aims. We investigate the dissipation of the bulk kinetic energy of a relativistic jet at different distances from the central power–house and analyse in detail how the dissipated energy is radiated away. Methods. We assume that the location of the dissipation region is a function of the bulk Lorentz factorΓof the jet, being closer to the center for smallerΓ. This assumption is naturally fulfilled in the internal shock scenario. The dissipated energy is partially used to accelerate electrons and to amplify the magnetic field. This process creates a source inside the jet (blob). Such blobs may efficiently produce synchrotron and inverse Compton emission. Results. We find that even if the blobs or shells responsible for the blazar activity carry the same energy (in bulk kinetic form), the fact that they move at differentΓcan produce dramatic variations in different bands, even if the bolometric luminosity is instead very similar. This is due to the relative importance of the synchrotron, self–Compton and external Compton radiation processes, which greatly changes by changingΓ and the compactness of the source, even if the total radiated energy is constant. We then find that the jet can produce most of its radiative output at small distances from the putative black–hole and its accretion disk, if this implies a low level of emitted MeV–GeV flux. Our findings, which we apply for illustrative purposes to the blazar 3C 454.3, will be easily testable by the comingγ–ray satellite, such as AGILE and GLAST.