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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 ᵢ, from the semirelativistic regime (ultrarelativistic electrons but nonrelativistic ions, 0. 001 >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 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-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 ᵢ and asymptotes to about 0. 25 in the semirelativistic regime; (4) reconnection leads to efficient nonthermal electron acceleration with a ᵢ-dependent power-law index, p (ᵢ) const+0. 7 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.
Werner et al. (Thu,) studied this question.