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

International audienceMagnetic 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 non-relativistic 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 non-thermal 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; and (4) reconnection leads to efficient non-thermal electron acceleration with a σ_i-dependent power-law index, $$p(\sigma _{\rm i})\simeq {\rm const}+0.7\sigma _{\rm i}^{-1/2}$$. These findings are important for understanding black hole systems and lend support to semirelativistic reconnection models for powering non-thermal emission in blazar jets, offering a natural explanation for the spectral indices observed in these systems