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.
Highlights
Blazars – active galactic nuclei (AGN) with a jetted relativistic outflow pointing towards us – are commonly classified according to their characteristic double-humped spectral energy distribution (SED; Urry & Padovani 1995; Fossati et al 1998)
Ten or so extreme-TeV sources are known to date (Biteau et al 2020), current estimates based on data from the Fermi-LAT γ-ray telescope predict about five times more candidate sources that need to be confirmed with future instruments in the TeV range (Costamante 2020)
The emission region described by the simple one-zone model may represent either a plasma blob moving with relativistic speed through the jet, into which accelerated particles are injected from a leading bow shock, or the region in the relativistic plasma flow downstream from a standing shock
Summary
Blazars – active galactic nuclei (AGN) with a jetted relativistic outflow pointing towards us – are commonly classified according to their characteristic double-humped spectral energy distribution (SED; Urry & Padovani 1995; Fossati et al 1998). Arriving at a satisfactory description of extreme-TeV sources in the standard one-zone leptonic framework seemingly requires two essential ingredients (e.g., Tavecchio et al 1998; Ghisellini et al 2002; Katarzynski et al 2006; Costamante et al 2018): (1) lower magnetic fields ( 10 mG) than for more common high-frequency-peaked BL Lac objects (HBLs), corresponding to magnetic field energy densities well below equipartition, and (2) a peculiar lepton distribution with most of the energy carried by particles of a large Lorentz factor, γe ∼ 103−104.
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