Particle acceleration in neutron star ultra-strong electromagnetic fields

TL;DR

This paper introduces a new particle pusher algorithm for ultra-strong electromagnetic fields around neutron stars, demonstrating particle acceleration up to extreme Lorentz factors and analyzing their trajectories and behaviors.

Contribution

The study presents a novel particle simulation method in neutron star magnetospheres and explores particle dynamics without full self-consistency, highlighting charge-dependent acceleration and trajectories.

Findings

Particles reach Lorentz factors up to 10^{14} for electrons.

Particle trajectories depend on charge-to-mass ratio and magnetic inclination.

Particles can be captured, trapped, or ejected from the neutron star environment.

Abstract

In this paper, we discuss the results of a new particle pusher in realistic ultra-strong electromagnetic fields as those encountered around rotating neutron stars. After presenting results of this algorithm in simple fields and comparing them to expected exact analytical solutions, we present new simulations for a rotating magnetic dipole in vacuum for a millisecond pulsar by using Deutsch solution. Particles are injected within the magnetosphere, neglecting radiation reaction, interaction among them and their feedback on the fields. Our simulations are therefore not yet fully self-consistent because Maxwell equations are not solved according to the current produced by these particles. The code highlights the symmetrical behaviour of particles of opposite charge to mass ratio $q/m$ with respect to the north and south hemispheres. The relativistic Lorentz factor of the accelerated particles is proportional to this ratio $q/m$: protons reach up to $\gamma_p \simeq 10^{10.7}$, whereas electrons reach up to $\gamma_e \simeq 10^{14}$. Our simulations show that particles could be either captured by the neutron star, trapped around it, or ejected far from it, well outside the light-cylinder. Actually, for a given charge to mass ratio, particles follow similar trajectories. These particle orbits show some depleted directions, especially at high magnetic inclination with respect to the rotation axis for positive charges and at low inclination for negative charges because of symmetry. Other directions are preferred and loaded with a high density of particles, some directions concentrating the highest or lowest acceleration efficiencies.