Electron-proton co-acceleration on relativistic shocks in extreme-TeV blazars

¹ Laboratoire Univers et Théories, Observatoire de Paris, Université PSL, CNRS, Université de Paris, 92190 Meudon, France
e-mail: andreas.zech@obspm.fr
² Institut d’Astrophysique de Paris, CNRS – Sorbonne Université, 98 bis boulevard Arago, 75014 Paris, France
e-mail: lemoine@iap.fr

Received: 12 April 2021
Accepted: 31 July 2021

Abstract

Aims. The multi-wavelength emission from a newly identified population of ‘extreme-TeV’ blazars, with Compton peak frequencies around 1 TeV, is difficult to interpret with standard one-zone emission models. Large values of the minimum electron Lorentz factor and quite low magnetisation values seem to be required.

Methods. We propose a scenario where protons and electrons are co-accelerated on internal or recollimation shocks inside the relativistic jet. In this situation, energy is transferred from the protons to the electrons in the shock transition layer, leading naturally to a high minimum Lorentz factor for the latter. A low magnetisation favours the acceleration of particles in relativistic shocks.

Results. The shock co-acceleration scenario provides additional constraints on the set of parameters of a standard one-zone lepto-hadronic emission model, reducing its degeneracy. Values of the magnetic field strength of a few mG and minimum electron Lorentz factors of 10³ to 10⁴, required to provide a satisfactory description of the observed spectral energy distributions of extreme blazars, result here from first principles. While acceleration on a single standing shock is sufficient to reproduce the emission of most of the extreme-TeV sources we have examined, re-acceleration on a second shock appears needed for those objects with the hardest γ-ray spectra. Emission from the accelerated proton population, with the same number density as the electrons but in a lower range of Lorentz factors, is strongly suppressed. Satisfactory self-consistent representations were found for the most prominent representatives of this new blazar class.