Particle Injection in 3D Relativistic Magnetic Reconnection

Magnetic reconnection in the relativistic regime (where σ, the magnetic energy per particle rest energy, is large) triggers a causal chain of particle injection followed by power-law formation. Particle injection, i.e., particle acceleration up to the injection energy γ_inj (the low-energy cutoff of the power-law spectrum), can be understood by answering two questions: (a) What is the value of the injection energy γ_inj, and why? (b) What mechanisms are primarily responsible for injecting particles? Using fully kinetic particle-in-cell simulations, we explore these questions and investigate how the results depend on the dimensionality (3D vs 2D) and σ. We find that 3D effects do not have a strong influence on the results, suggesting that (i) 3D-only acceleration mechanisms identified in previous works may only apply to already injected particles and (ii) the results of 2D injection studies are extendable to 3D. When scanning from σ = 8 to σ = 64, our measurements suggest that γ_inj ~ sqrt(σ), contrary to the traditional expectation of γ_inj ~ σ. Assuming that the high-energy power-law cutoff scales as γ_c ~ σ, the dynamic range R = γ_c/γ_inj of the power law scales as R ~ sqrt(σ). This suggests that reconnection-powered high-energy flares in relativistic astrophysical sources may have broader photon spectra than previously anticipated.