2009MNRAS.397..985G

The jets of powerful blazars propagate within relatively dense regions of radiation produced externally to the jet. This radiation is a key ingredient to understand the origin of the high-energy emission of blazars, from the X-ray to the γ-ray energy band. The main components contributing to the external radiation field are the accretion disc emission, including its X-ray corona, the broad-line region, the infrared emitting torus and the cosmic background radiation. Their importance changes as a function of the distance from the black hole and of the value of the bulk Lorentz factor of the jet. These external radiation fields control the amount of the inverse Compton radiation with respect to the synchrotron flux. Therefore, the predicted spectral energy distribution (SED) will depend on where the jet dissipates part of its energy to produce the observed radiation. We investigate in detail how the SED changes as a function of the location of the jet dissipation region by assuming rather `standard' (i.e. `canonical') prescriptions for the accretion disc and its X-ray corona, the profile of the jet magnetic field and the external radiation. We confirm that most of the dissipation, if producing the γ-ray flux we see, must occur at hundreds of Schwarzschild radii from the black hole, to avoid the γ - γ ⟶ e^± process, and the consequent re-emission by the produced pairs. The magnetic energy density of a `canonical' jet almost never dominates the radiative cooling of the emitting electrons, and consequently the inverse Compton flux almost always dominates the bolometric output. This is more so for large black hole masses. Dissipation taking place beyond the broad-line region is particularly interesting, since it accounts in a simple way for the largest inverse Compton to synchrotron flux ratios accompanied by an extremely hard X-ray spectrum. Furthermore, it makes the high-power blazars at high redshift useful tools to study the optical to UV cosmic backgrounds.