Electron and Proton Acceleration in Trans-relativistic Magnetic Reconnection: Dependence on Plasma Beta and Magnetization

Nonthermal electron acceleration via magnetic reconnection is thought to play an important role in powering the variable X-ray emission from radiatively inefficient accretion flows around black holes. The trans-relativistic regime of magnetic reconnection—where the magnetization σ , defined as the ratio of magnetic energy density to enthalpy density, is ∼1—is frequently encountered in such flows. By means of a large suite of two-dimensional particle-in-cell simulations, we investigate electron and proton acceleration in the trans-relativistic regime. We focus on the dependence of the electron energy spectrum on σ and the proton β (the ratio of proton thermal pressure to magnetic pressure). We find that the electron spectrum in the reconnection region is nonthermal and can be modeled as a power law. At low β , the slope, p , is independent of β and hardens with increasing σ as p ≃ 1.8 + 0.7 / σ . Electrons are primarily accelerated by the nonideal electric field at X-points, either in the initial current layer or in current sheets generated between merging magnetic islands. At higher values of β , the electron power law steepens, and the electron spectrum eventually approaches a Maxwellian distribution for all values of σ . At values of β near β max ≈ 1/4 σ , when both electrons and protons are relativistically hot prior to reconnection, the spectra of both species display an additional component at high energies, containing a few percent of particles. These particles are accelerated via a Fermi-like process by bouncing between the reconnection outflow and a stationary magnetic island