Particle Acceleration in Relativistic Magnetized Collisionless Pair Shocks: Dependence of Shock Acceleration on Magnetic Obliquity

TL;DR

This study uses particle-in-cell simulations to analyze how magnetic obliquity affects particle acceleration in relativistic magnetized collisionless pair shocks, revealing that efficient acceleration occurs only in specific subluminal configurations.

Contribution

It provides a detailed analysis of shock structure and particle acceleration dependence on magnetic obliquity, highlighting the conditions for efficient acceleration in relativistic pair shocks.

Findings

Efficient particle acceleration occurs only in subluminal shocks with certain magnetic inclinations.

The downstream spectrum includes a Maxwellian and a high-energy power-law tail with exponential cutoff.

Acceleration mechanisms switch from Diffusive Shock Acceleration to Shock-Drift Acceleration as inclination increases.

Abstract

We investigate shock structure and particle acceleration in relativistic magnetized collisionless pair shocks by means of 2.5D and 3D particle-in-cell simulations. We explore a range of inclination angles between the pre-shock magnetic field and the shock normal. We find that only magnetic inclinations corresponding to "subluminal" shocks, where relativistic particles following the magnetic field can escape ahead of the shock, lead to particle acceleration. The downstream spectrum in such shocks consists of a relativistic Maxwellian and a high-energy power-law tail with exponential cutoff. For increasing magnetic inclination in the subluminal range, the high-energy tail accounts for an increasing fraction of particles (from ~1% to ~2%) and energy (from ~4% to ~12%). The spectral index of the power law increases with angle from -2.8+-0.1 to -2.3+-0.1. Particle energization is driven by the Diffusive Shock Acceleration process for nearly parallel shocks, and switches to Shock-Drift Acceleration for larger subluminal inclinations. For "superluminal" shocks, the downstream particle spectrum does not show any significant suprathermal tail. As seen from the upstream frame, efficient acceleration in relativistic (Lorentz factor gamma0 > 5) magnetized (sigma > 0.03) flows exists only for a very small range of magnetic inclination angles (< 34/gamma0 degrees), so relativistic astrophysical pair shocks have to be either nearly parallel or weakly magnetized to generate nonthermal particles. These findings place constraints on the models of AGN jets, Pulsar Wind Nebulae and Gamma Ray Bursts that invoke particle acceleration in relativistic magnetized shocks. (Abridged)