Fast Particle Acceleration in Threedimensional Relativistic Reconnection
Abstract
Magnetic reconnection is invoked as one of the primary mechanisms to produce energetic particles. We employ largescale 3D particleincell simulations of reconnection in magnetically dominated (σ = 10) pair plasmas to study the energization physics of highenergy particles. We identify an acceleration mechanism that only operates in 3D. For weak guide fields, 3D plasmoids/flux ropes extend along the zdirection of the electric current for a length comparable to their crosssectional radius. Unlike in 2D simulations, where particles are buried in plasmoids, in 3D we find that a fraction of particles with γ ≳ 3σ can escape from plasmoids by moving along z, and so they can experience the largescale fields in the upstream region. These "free" particles preferentially move in z along Speiserlike orbits sampling both sides of the layer and are accelerated linearly in timetheir Lorentz factor scales as γ ∝ t, in contrast to $\gamma \propto \sqrt{t}$ in 2D. The energy gain rate approaches ~eE _{rec} c, where E _{rec} ≃ 0.1B _{0} is the reconnection electric field and B _{0} the upstream magnetic field. The spectrum of free particles is hard, ${{dN}}_{\mathrm{free}}/d\gamma \propto {\gamma }^{1.5}$ , contains ~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 ultrahighenergy cosmic rays.
 Publication:

The Astrophysical Journal
 Pub Date:
 December 2021
 DOI:
 10.3847/15384357/ac2e08
 arXiv:
 arXiv:2105.00009
 Bibcode:
 2021ApJ...922..261Z
 Keywords:

 739;
 Astrophysics  High Energy Astrophysical Phenomena
 EPrint:
 14 pages, 10 figures, 1 table, submitted to ApJ