  We investigate the impact on the properties of high-energy emitting BL Lac
objects of a population of electrons with an anisotropic momentum distribution.
We adopt a simple phenomenological description of the anisotropy, in which the
most energetic electrons have a small pitch angle and the least energetic
electrons are isotropic, as proposed by Sobacchi \& Lyubarsky (2019). We
explore (i) a simple model that assumes a phenomenological shape for the
electron energy distribution, and (ii) a self-consistent scheme in which the
electrons follow a distribution which is the result of the balance between
injection and radiative losses (we include the effects of the anisotropy on the
synchrotron cooling rate). Considering the BL Lac object Mkn 421 as
representative of the entire class, we show that in both cases the emission can
be satisfactorily reproduced under equipartition between the magnetic field and
the relativistic electrons. This is in better agreement with the idea that jets
are launched as Poynting dominated flows with respect to the standard isotropic
scenario, which requires both a low magnetization and a low radiative
efficiency to reproduce the observed emission. The hard spectrum predicted for
the inverse Compton continuum at TeV energies could be used as a potential test
of the anisotropic model.
