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

TL;DR

This study investigates how magnetic reconnection in relativistic electron-ion plasmas accelerates particles and affects energy distribution, revealing key dependencies on ion magnetization and implications for astrophysical phenomena.

Contribution

It provides a comprehensive PIC simulation analysis of relativistic electron-ion reconnection across various magnetization regimes, highlighting new insights into energy partitioning and particle acceleration.

Findings

Reconnection rate is approximately 0.1 of the Alfvenic rate across regimes.

Electrons form relativistic outflows even at low magnetization.

Energy partitioning shifts from equal in ultrarelativistic to about 25% electron energy in semirelativistic regimes.

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 $\sigma_i$, from the semirelativistic regime (ultrarelativistic electrons but nonrelativistic ions, 0.001<<$\sigma_i$<<1) to the fully relativistic regime (both species are ultrarelativistic, $\sigma_i$>>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 $\sigma_i$. 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-$\sigma_i$ 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 $\sigma_i$ and asymptotes to about 0.25 in the semirelativistic regime; (4) reconnection leads to efficient nonthermal electron acceleration with a $\sigma_i$-dependent power-law index, $p(\sigma_i) \simeq $const$+0.7 {\sigma_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.