Fermi~I particle acceleration in converging flows mediated by magnetic reconnection

TL;DR

This paper investigates a novel particle acceleration mechanism in magnetic reconnection layers within converging flows, highlighting conditions for efficient acceleration and potential astrophysical sites, with implications for high-energy cosmic rays.

Contribution

It introduces a new acceleration process driven by magnetic reconnection in converging flows, providing analytical estimates and identifying specific astrophysical scenarios where it may operate.

Findings

Acceleration occurs if reconnection layer is thin and magnetic field is disordered.

Electrons may be limited by synchrotron losses.

Acceleration is faster and produces harder spectra than diffusive shock acceleration.

Abstract

Context. Converging flows with strong magnetic fields of different polarity can accelerate particles through magnetic reconnection. If the particle mean free path is longer than the reconnection layer is thick, but much shorter than the entire reconnection structure, the particle will mostly interact with the incoming flows potentially with a very low escape probability. Aims. We explore, in general and also in some specific scenarios, the possibility of particles to be accelerated in a magnetic reconnection layer by interacting only with the incoming flows. Methods. We characterize converging flows that undergo magnetic reconnection, and derive analytical estimates for the particle energy distribution, acceleration rate, and maximum energies achievable in these flows. We also discuss a scenario, based on jets dominated by magnetic fields of changing polarity, in which this mechanism may operate. Results. The proposed acceleration mechanism operates if the reconnection layer is much thinner than its transversal characteristic size, and the magnetic field has a disordered component. Synchrotron losses may prevent electrons from entering in this acceleration regime. The acceleration rate should be faster, and the energy distribution of particles harder than in standard diffusive shock acceleration. The interaction of obstacles with the innermost region of jets in active galactic nuclei and microquasars may be suitable sites for particle acceleration in converging flows.