# The Mechanism of Electron Injection and Acceleration in Trans-Relativistic Reconnection

## TL;DR

This study uses particle-in-cell simulations to explore how electron injection and acceleration occur during trans-relativistic magnetic reconnection, revealing the roles of X-points, plasmoids, and electric fields in high-energy astrophysical phenomena.

## Contribution

It provides new insights into the mechanisms of electron acceleration in trans-relativistic reconnection, highlighting the importance of X-points and electric field components in shaping electron energy distributions.

## Key findings

- More X-points increase acceleration efficiency.
- Non-ideal electric fields at X-points initiate acceleration.
- Out-of-plane electric fields influence high-energy tail hardness.

## Abstract

Electron acceleration during magnetic reconnection is thought to play a key role in time-variable high-energy emission from astrophysical systems. By means of particle-in-cell simulations of trans-relativistic reconnection, we investigate electron injection and acceleration mechanisms in low-$\beta$ electron-proton plasmas. We set up a diversity of density and field structures (e.g., X-points and plasmoids) by varying the guide field strength and choosing whether to trigger reconnection or let it spontaneously evolve. We show that the number of X-points and plasmoids controls the efficiency of electron acceleration, with more X-points leading to a higher efficiency. Using on-the-fly acceleration diagnostics, we also show that the non-ideal electric fields associated with X-points play a critical role in the first stages of electron acceleration. As a further diagnostic, we include two populations of test particles that selectively experience only certain components of electric fields. We find that the out-of-plane component of the parallel electric field determines the hardness of the high-energy tail of the electron energy distribution. These results further our understanding of electron acceleration in this regime of magnetic reconnection and have implications for realistic models of black hole accretion flows.