On the magnetization and the radiative efficiency of BL Lac jets

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 (U_e ∼ U_B), which is likely the case if the magnetic fields play a dynamically important role in the jet’s acceleration process; (ii) cool efficiently in a dynamical time (t_cool t_dyn), which is suggested by the fact that a large fraction of the jet’s kinetic energy is promptly converted into radiation. 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. Indeed, the model predicts U_e U_B and t_cool t_dyn for most of the objects. Here, we closely investigate one of the key assumptions of this model, namely that the momentum distribution of the non-thermal electrons is isotropic. We find that this assumption may be an oversimplification. If the magnetic energy is dissipated via a turbulent MHD cascade, the highest energy electrons may instead retain a small pitch angle. Since the synchrotron emissivity is suppressed when the pitch angle is small, this effect may importantly affect the modelling of the SED. As an illustrative example, we present an anisotropic model for the electron momentum distribution such that U_e ∼ U_B and t_cool t_dyn at the same time. Our model manages to simultaneously solve the two problems with one only more free parameter with respect to the usual isotropic one-zone SSC model.