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

TL;DR

This paper proposes a co-acceleration mechanism for electrons and protons in relativistic shocks within extreme-TeV blazars, explaining their spectral features and constraining emission model parameters from first principles.

Contribution

It introduces a shock co-acceleration scenario that naturally explains high minimum electron Lorentz factors and low magnetization, reducing degeneracy in emission models for extreme blazars.

Findings

Co-acceleration explains high electron Lorentz factors (10^3-10^4).

Low magnetization favors relativistic shock acceleration.

Re-acceleration needed for the hardest gamma-ray spectra.

Abstract

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. 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. 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 10^3 to 10^4, 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 gamma-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.