Non-thermal particle acceleration in collisionless relativistic electron–proton reconnection

Abstract

Magnetic reconnection in relativistic collisionless plasmas can accelerate particles and power high-energy emission in various astrophysical systems. Whereas most previous studies focused on relativistic reconnection in pair plasmas, less attention has been paid to electron-ion plasma reconnection, expected in black hole accretion flows and relativistic jets. We report a comprehensive particle-in-cell numerical investigation of reconnection in an electron-ion plasma, spanning a wide range of ambient ion magnetizations σ i , from the semirelativistic regime (ultrarelativistic electrons but nonrelativistic ions, 10 −3 σ i 1) to the fully relativistic regime (both species are ultrarelativistic, σ i 1). We investigate how the reconnection rate, electron and ion plasma flows, electric and magnetic field structures, electron/ion energy partitioning, and nonthermal particle acceleration depend on σ i . Our key findings are: (1) the reconnection rate is about 0.1 of the Alfvénic rate across all regimes; (2) electrons can form concentrated moderately relativistic outflows even in the semirelativistic, small-σ i regime; (3) while the released magnetic energy is partitioned equally between electrons and ions in the ultrarelativistic limit, the electron energy fraction declines gradually with decreased σ i and asymptotes to about 0.25 in the semirelativistic regime; (4) reconnection leads to efficient nonthermal electron acceleration with a σ i -dependent power-law index, p(σ i ) const + 0.7σ −1/2 i . These findings are important for understanding black hole systems and lend support to semirelativistic reconnection models for powering nonthermal emission in blazar jets, offering a natural explanation for the spectral indices observed in these systems.

Inferring the jet parameters of active galactic nuclei using Bayesian analysis of VLBI data with a non-uniform jet model

“…Magnetic reconnection in a both normal and pairs plasma results in various electrons spectra -from s ≈ 1.0 to to s ≈ 4 depending on magnetization σ and value of the guide magnetic field (Sironi & Spitkovsky 2014;Guo et al 2015Guo et al , 2016Werner & Uzdensky 2017;Werner et al 2018). It however requires σ 1 to be efficient (Werner et al 2018). Obtained values ( Figure 10) are thus consistent with magnetic reconnection only for e − /e + jet and γ min ≥ 100.…”

Two-temperature, Magnetically Arrested Disc simulations of the jet from the supermassive black hole in M87

“…This model of the dissipation scale of plasma turbulence motivates our second heating prescription, obtained by measuring electron heating in particle-in-cell (PIC) simulations of transrelativistic reconnection. Rowan et al (2017) explored magnetic reconnection with PIC simulations in plasmas over a trans-relativistic range of temperatures and magnetic field strengths (see also Werner et al 2018 for a similar study). A fitting function to δe as measured in the reconnection PIC simulations of Rowan et al (2017) was presented in Chael et al (2018a).…”

Effects of Synchrotron Cooling and Pair Production on Collisionless Relativistic Reconnection

“…We set up a 2D relativistic particle-in-cell simulation of a reconnecting current sheet using the code TRISTAN-MP (Spitkovsky 2005). The current sheet is initially in Harris equilibrium and is perturbed by either the artificial cooling of the central region or by adding a temporary magnetic loop near the center (see, e.g., Werner et al 2018b). In either case, the evolution does not depend on the way we trigger reconnection, and the later steady state behavior is completely determined by our initial parameters.…”