Particle Injection and Nonthermal Particle Acceleration in Relativistic Magnetic Reconnection

TL;DR

This study uses kinetic simulations to explore how guide-field strength and domain size influence particle acceleration mechanisms and spectral features in relativistic magnetic reconnection, relevant to high-energy astrophysical phenomena.

Contribution

It identifies the roles of different electric field components and acceleration mechanisms, revealing how guide-field strength and domain size affect particle energization and spectral properties.

Findings

Stronger guide fields increase the power-law index and injection energy.

Fermi and pickup processes dominate injection in weak guide fields.

Parallel electric fields are crucial for injection in strong guide fields.

Abstract

Magnetic reconnection in the relativistic regime has been proposed as an important process for the efficient production of nonthermal particles and high-energy emissions. Using fully kinetic particle-in-cell simulations, we investigate how guide-field strength and domain size affect characteristic spectral features and acceleration processes. We study two stages of acceleration: energization up until the injection energy $\gamma_{\rm inj}$ and further acceleration that generates a power-law spectrum. Stronger guide fields increase the power-law index and $\gamma_{\rm inj}$, which suppresses acceleration efficiency. These quantities seemingly converge with increasing domain size, suggesting that our findings can be extended to large-scale systems. We find that three distinct mechanisms contribute to acceleration during injection: particle streaming along the parallel electric field, Fermi reflection, and the pickup process. Fermi and pickup processes, related to the electric field perpendicular to the magnetic field, govern the injection for weak guide fields and larger domains. Meanwhile, parallel electric fields are important for injection in the strong guide field regime. In the post-injection stage, we find that perpendicular electric fields dominate particle acceleration in the weak guide field regime, whereas parallel electric fields control acceleration for strong guide fields. These findings will help explain the nonthermal acceleration and emissions in high-energy astrophysics, including black hole jets and pulsar wind nebulae.