On the magnetisation and the radiative efficiency of BL Lac jets

Understand

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 ($U_e\sim U_B$), 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 ($t_{\rm cool}\lesssim t_{\rm dyn}$), 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 $U_e\gg U_B$ and $t_{\rm cool}\gg t_{\rm dyn}$ 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.