Fast Particle Acceleration in Three-dimensional Relativistic Reconnection
Hao Zhang, Lorenzo Sironi, Dimitrios Giannios
Abstract
Abstract Magnetic reconnection is invoked as one of the primary mechanisms to produce energetic particles. We employ large-scale 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 an 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 ∝ t , in contrast to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>γ</mml:mi> <mml:mo>∝</mml:mo> <mml:msqrt> <mml:mrow> <mml:mi>t</mml:mi> </mml:mrow> </mml:msqrt> </mml:math> in 2D. The energy gain rate approaches ∼ eE 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, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi mathvariant="italic">dN</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>free</mml:mi> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi>d</mml:mi> <mml:mi>γ</mml:mi> <mml:mo>∝</mml:mo> <mml:msup> <mml:mrow> <mml:mi>γ</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1.5</mml:mn> </mml:mrow> </mml:msup> </mml:math> , 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 ultra-high-energy cosmic rays.