Applying Relativistic Reconnection to Blazar Jets

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.