Guide field effects on particle acceleration in three-dimensional relativistic magnetic reconnection

Abstract

Relativistic magnetic reconnection is a key mechanism for particle acceleration in astrophysical plasmas. Recent 3D particle-in-cell simulations have shown that, for weak guide fields, particles can be accelerated at nearly the maximal rate by escaping the reconnected plasma and gaining energy from the large-scale upstream electric field. Here, we investigate whether this fast acceleration channel persists as the guide field becomes comparable to the reconnecting field. We perform 3D simulations of relativistic pair-plasma reconnection, complemented by corresponding 2D runs, using open boundaries along the reconnection outflows to establish a quasi-steady state. We find that increasing the guide field strength reduces the reconnection rate and causes energetic particles to become more closely aligned with the local magnetic field. In 3D, the fast acceleration channel---absent in 2D---persists across the full range of guide fields investigated, but involves a progressively smaller fraction of particles as the guide field increases. Consequently, stronger guide fields produce steeper particle energy spectra and lower maximum Lorentz factors.