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

G. R. Werner; D. A. Uzdensky; M. C. Begelman; B. Cerutti; K. Nalewajko

doi:10.1093/mnras/stx2530

paper

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

arXiv:1612.04493 · doi:10.1093/mnras/stx2530

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, 0.001<< $σ_{i}$ <<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 Alfvenic 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 $+ 0.7 σ_{i}^{- 1/2}$ . 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.

arXiv comment30 pages, 26 figures

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

Abstract

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