Relativistic Nonthermal Particle Acceleration in Plasmoid-Mediated Magnetic Reconnection*

Relativistic magnetic reconnection has been shown numerically to be an efficient and powerful energetic particle accelerator, providing an appealing physical explanation for nonthermal high-energy radiation observed in many astrophysical sources. Here, I present an analytical model aiming to elucidate the key physical processes responsible for relativistic nonthermal particle acceleration (NTPA) by collisionless reconnection in the large-system, plasmoid-mediated regime, and to explain the main features of reconnection-driven NTPA seen in simulations. These features include the dependence of the power-law index α and high-energy cutoff γ_c of the resulting nonthermal particle energy spectrum f(γ) on the ambient plasma magnetization and guide magnetic field, and (for γ_c) on the system size.  In this model, energetic particles are continuously accelerated by the main reconnection electric field E_rec until they become magnetized by the reconnected magnetic field and eventually trapped in plasmoids large enough to confine them. I argue that the balance between electric acceleration and magnetization controls the power-law index, while trapping by plasmoids governs γ_c, thus tying the particle energy spectrum to the plasmoid distribution function.