Abstract
Rapid and luminous flares of non-thermal radiation observed in blazars require an efficient mechanism of energy dissipation and particle acceleration in relativistic active galactic nuclei (AGN) jets. Particle acceleration in relativistic magnetic reconnection is being actively studied by kinetic numerical simulations. Relativistic reconnection produces hard power-law electron energy distributions N ( γ ) ∝ γ − p exp ( − γ / γ max ) with index p → 1 and exponential cut-off Lorentz factor γ max ∼ σ in the limit of magnetization σ = B 2 / ( 4 π w ) ≫ 1 (where w is the relativistic enthalpy density). Reconnection in electron-proton plasma can additionally boost γ max by the mass ratio m p / m e . Hence, in order to accelerate particles to γ max ∼ 10 6 in the case of BL Lacs, reconnection should proceed in plasma of very high magnetization σ max ≳ 10 3 . On the other hand, moderate mean jet magnetization values are required for magnetic bulk acceleration of relativistic jets, σ mean ∼ Γ j ≲ 20 (where Γ j is the jet bulk Lorentz factor). I propose that the systematic dependence of γ max on blazar luminosity class—the blazar sequence—may result from a systematic trend in σ max due to homogeneous loading of leptons by pair creation regulated by the energy density of high-energy external radiation fields. At the same time, relativistic AGN jets should be highly inhomogeneous due to filamentary loading of protons, which should determine the value of σ mean roughly independently of the blazar class.
Highlights
Blazars are persistent extragalactic sources of non-thermal radiation extending from the radio to the gamma-ray band and characterized by stochastic variability over a wide range of time scales.Multiwavelength observations of blazars typically reveal two major broad spectral components, with the low-energy one interpreted universally as synchrotron radiation of electrons.High-resolution interferometric radio/mm observations reveal a core-jet structure with individual jet substructures propagating with apparently superluminal velocity [1,2]
Blazars are associated with active galactic nuclei (AGN) equipped with jets, with one of the jets pointing closely at the observer [4], which leads to a dramatic relativistic boost of apparent luminosity [5]
Blazars are typically classified into more luminous flat-spectrum radio quasars (FSRQs) and less luminous BL Lac objects (BL Lacs)
Summary
Blazars are persistent extragalactic sources of non-thermal radiation extending from the radio to the gamma-ray band and characterized by stochastic variability over a wide range of time scales. Multiwavelength observations of blazars typically reveal two major broad spectral components, with the low-energy one (radio—UV/X-ray) interpreted universally as synchrotron radiation of electrons. High apparent luminosities of blazars, up to Lγ ∼ 1050 erg · s−1 in the γ-ray band [7], require efficient in situ dissipation of the jet power, efficient particle acceleration and efficient radiative mechanisms. Modeling of the spectral energy distributions of blazars can be performed in two basic scenarios [8]: in the leptonic scenario, the high-energy spectral component is interpreted as inverse Compton scattering of soft radiation fields by energetic electrons; in the hadronic scenario, it is interpreted as due to various mechanisms involving relativistic protons. Can relativistic magnetic reconnection explain energy dissipation and particle acceleration in blazar jets?.
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