Fast particle acceleration in three-dimensional relativistic reconnection

Hao Zhang; Lorenzo Sironi; Dimitrios Giannios

doi:10.3847/1538-4357/ac2e08

paper

Fast particle acceleration in three-dimensional relativistic reconnection

arXiv:2105.00009 · doi:10.3847/1538-4357/ac2e08

Abstract

Magnetic reconnection is invoked as one of the primary mechanisms to produce energetic particles. We employ large-scale three-dimensional (3D) particle-in-cell simulations of reconnection in magnetically-dominated ( $σ = 10$ ) pair plasmas to study the energization physics of high-energy particles. We identify a novel acceleration mechanism that only operates in 3D. For weak guide fields, 3D plasmoids / flux ropes extend along the $z$ direction of the electric current for a length comparable to their cross-sectional radius. Unlike in 2D simulations, where particles are buried in plasmoids, in 3D we find that a fraction of particles with $γ ≳ 3 σ$ can escape from plasmoids by moving along $z$ , and so they can experience the large-scale fields in the upstream region. These "free" particles preferentially move in $z$ along Speiser-like orbits sampling both sides of the layer, and are accelerated linearly in time -- their Lorentz factor scales as $γ \propto t$ , in contrast to $γ \propto t$ in 2D. The energy gain rate approaches $\sim e E_{rec} c$ , where $E_{rec} ≃ 0.1 B_{0}$ is the reconnection electric field and $B_{0}$ the upstream magnetic field. The spectrum of free particles is hard, $d N_{free} / d γ \propto γ^{- 1.5}$ , contains $\sim 20%$ of the dissipated magnetic energy independently of domain size, and extends up to a cutoff energy scaling linearly with box size. Our results demonstrate that relativistic reconnection in GRB and AGN jets may be a promising mechanism for generating ultra-high-energy cosmic rays.

14 pages, 10 figures, 1 table, submitted to ApJ

Fast particle acceleration in three-dimensional relativistic reconnection

Abstract

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