On the magnetisation and the radiative efficiency of BL Lac jets. (arXiv:1812.11435v1 [astro-ph.HE])
<a href="http://arxiv.org/find/astro-ph/1/au:+Sobacchi_E/0/1/0/all/0/1">E. Sobacchi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Lyubarsky_Y/0/1/0/all/0/1">Y.E. Lyubarsky</a>
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 ($U_esim U_B$), 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 $U_egg U_B$ 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. 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 $U_esim U_B$ and $t_{rm cool}lesssim t_{rm dyn}$ 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.
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 ($U_esim U_B$), 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 $U_egg U_B$ 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. 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 $U_esim U_B$ and $t_{rm cool}lesssim t_{rm dyn}$ 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.
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