Extreme blazars: the result of unstable recollimated jets?

Extreme BL Lacs (EHBL) form a subclass of blazars, which challenge standard emission scenarios. In a recent study it has been argued that their peculiar properties can be explained if emitting electrons are accelerated in a series of oblique shocks induced by the recollimation of the relativistic jet. However, new 3 D simulations of recollimated, weakly magnetized jets reveal that, in correspondence with the first recollimation shock, the flow develops a rapidly growing instability, becomes highly turbulent and decelerates, effectively hampering the formation of the multiple shock structure routinely observed in 2 D simulations. Building on these new findings, we propose here a revised scenario for EHBL, in which the emission is produced by electrons accelerated at the recollimation shock and subsequently further energized through stochastic acceleration in the turbulent downstream flow. We apply a simple version of this scenario to the prototypical EHBL 1ES 0229-200, showing that the spectral energy distribution (SED) can be satisfactorily reproduced with standard values of the main physical parameters.