Sketch of the scenario discussed in the text. The jet has a parabolic shape and carries a helical magnetic field. Particles are accelerated at some distance (z₀) from the black hole. Electrons, which have a very short cooling time, produce the optical emission in a thin shell close to the acceleration zone. Protons, instead, are in the slow-cooling regime and are advected by the flow up to a distance Δz ∼ z₀ (determined by the adiabatic energy losses). Protons produce the high-energy component of the SED, including the X-rays.