Magnetic Reconnection in 3D relativistic and semirelativistic astrophysical plasmas

Magnetic reconnection releases magnetic field energy, resulting in plasma heating and nonthermal particle acceleration (NTPA). Reconnection in astrophysical current sheets with relativistic collisionless plasma, e.g., near black holes or neutron stars, may play a key role energizing electrons that emit observable radiation. In this regime, simulations have shown that 2D reconnection yields substantial NTPA, but it remains an outstanding problem whether similar mechanisms operate in 3D. Using large 3D particle-in-cell simulations, we show that there is a regime (in relativistic pair plasma with ambient plasma beta near unity) where current sheet dynamics differ significantly from classic 2D reconnection due to dissolution of flux ropes and competition from the nonlinear drift-kink instability. Despite this, NTPA is seen to be surprisingly robust. We then expand our study to 3D semirelativistic electron-ion plasmas where electrons are relativistic but ions are not, in particular measuring the electrons/ion energy partition, which has important consequences for radiative signatures, e.g., of X-ray and gamma-ray flares from accreting black holes.