Extreme blazars: the result of unstable recollimated jets?

TL;DR

This paper revises the understanding of extreme BL Lac objects by incorporating 3D simulation insights, proposing a model where electrons are accelerated at the recollimation shock and further energized in turbulent downstream flow, successfully reproducing observed spectra.

Contribution

It introduces a new scenario for EHBL emission based on 3D simulation results, emphasizing stochastic acceleration in turbulent downstream regions.

Findings

The flow becomes turbulent and decelerates after the first recollimation shock in 3D simulations.

A revised emission model can reproduce the spectral energy distribution of 1ES 0229-200.

Turbulent downstream flow plays a significant role in electron energization for EHBLs.

Abstract

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 3D 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 2D 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 SED can be satisfactorily reproduced with standard values of the main physical parameters.