Spectra and power of blazar jets

The spectra of blazars form a sequence which can be parametrized in term of their observed bolometric luminosity. At the most powerful extreme of the sequence we find objects whose jet power can rival the power extracted by accretion, while at the low power end of the sequence we find TeV and TeV candidate blazars, whose spectral properties can give information on the particle acceleration mechanism and can help to measure the IR background. Most of the emission of blazars is produced at a distance of few hundreds Schwarzschild radii from the center, and must be a small fraction of the kinetic power carried by the jet itself: most of the jet energy must be transported outwards, to power the extended radio structures. The radiation produced by jets can be the result of internal shocks between shells of plasma with different bulk Lorentz factors. This mechanism, thought to be at the origin of the gamma–rays observed in gamma–ray bursts, can work even better in blazars, explaining the observed main characteristics of these objects. Chandra very recently discovered large scale X–ray jets both in blazars and in radio–galaxies, which can be explained by enhanced Compton emission with respect to a pure synchrotron self Compton model. In fact, besides the local synchrotron emission, the radiation coming from the sub–pc jet core and the cosmic background radiation can provide seed photons for the scattering process, enahancing the large scale jet X–ray emission in radio–galaxies and blazars, respectively.