| Abstract: | Theoretical modelling and observations of AGN jets suggest that the non-thermal electrons emitting the observed radiation should (i) carry an amount of energy comparable to the magnetic fields; (ii) cool efficiently in a dynamical time. These expectations are at odds with the results of the simplest one-zone Self-Synchro-Compton (SSC) model for the Spectral Energy Distribution (SED) of BL Lacs, which motivates a careful investigation of the model's assumptions. I will show that one of the key assumptions of the SSC model, namely that the momentum distribution of the non-thermal electrons is isotropic, may be wrong. If the magnetic energy is dissipated via a turbulent MHD cascade, the highest energy electrons may instead retain a small pitch angle. Since for small pitch angles the synchrotron emissivity is suppressed, this effect may importantly affect the modelling of the SED. As an illustrative example, I will present an anisotropic model for the electron momentum distribution such that at the same time (i) there is energy equipartition between the non-thermal electrons and the magnetic fields, and (ii) the non-thermal electrons cool efficiently in a dynamical time. |