Fermi-type particle acceleration from magnetic reconnection at the termination shock of a relativistic striped wind

TL;DR

This study uses advanced PIC simulations to demonstrate that magnetic reconnection at the termination shock of a relativistic striped wind efficiently accelerates particles to high energies, producing power-law spectra consistent with observed gamma-ray emissions.

Contribution

It extends PIC simulations to ultra-relativistic regimes, revealing the dominant Fermi-type acceleration mechanism and the scalability of particle energization in pulsar wind nebulae.

Findings

Magnetic reconnection efficiently converts magnetic energy to particle kinetic energy.

Particle spectra form a power-law distribution when bclambda/d_ebclarge.

Maximum electron energies can reach hundreds of TeV, explaining high-energy gamma-ray observations.

Abstract

An oblique-rotating pulsar generates a relativistic striped wind in a pulsar wind nebula (PWN). The termination shock of the PWN compresses the Poynting-flux-dominated flow and drives magnetic reconnection. By carrying out particle-in-cell (PIC) simulations of the termination shock of the PWN, we study the shock structure as well as the energy conversion processes and particle acceleration mechanisms. With the recent advances in the numerical methods, we extend the simulations to the ultra-relativistic regime with bulk Lorentz factor up to \gamma_{0}=10^{6}. Magnetic reconnection at the termination shock is highly efficient at converting magnetic energy to particle kinetic energy and accelerating particles to high energies. We find that the resulting energy spectra crucially depend on \lambda/d_{e}. When \lambda/d_{e} is large (\lambda\gtrsim40d_{e}) , the downstream particle spectra form a power-law distribution in the magnetically dominated relativistic wind regime with upstream magnetization parameter \sigma_{0}=10. By analyzing particle trajectories and statistical quantities relevant to particle energization, we find that Fermi-type mechanism dominates the particle acceleration and power-law formation. We find that the results for particle acceleration are scalable as \gamma_{0} and \sigma_{0} increase to large values. The maximum energy for electrons and positrons can reach hundreds of TeV if the wind has a bulk Lorentz factor \gamma_{0}\approx10^{6} and magnetization parameter \sigma_{0}=10, which can explain the recent observations of high-energy gamma-rays from pulsar wind nebulae (PWNe).