Diffusive shock acceleration in relativistic, oblique shocks

TL;DR

This paper investigates whether relativistic, oblique shocks can accelerate cosmic rays via diffusive shock acceleration, using combined PIC and MHD simulations, and finds that high Mach number shocks can indeed accelerate particles.

Contribution

It introduces a novel simulation approach combining PIC and MHD methods to study particle acceleration in relativistic, oblique shocks, revealing conditions for effective acceleration.

Findings

Relativistic, oblique shocks can accelerate particles if the Alfvenic Mach number is sufficiently high.

The combined PIC and MHD simulation approach effectively models particle acceleration over larger scales.

Quasi-perpendicular shocks are capable of diffusive shock acceleration under certain conditions.

Abstract

Cosmic rays are charged particles that are accelerated to relativistic speeds by astrophysical shocks. Numerical models have been successful in confirming the acceleration process for (quasi-)parallel shocks, which have the magnetic field aligned with the direction of the shock motion. However, the process is less clear when it comes to (quasi-)perpendicular shocks, where the field makes a large angle with the shock-normal. For such shocks, the angle between the magnetic field and flow ensures that only highly energetic particles can travel upstream at all, reducing the upstream current. This process is further inhibited for relativistic shocks, since the shock can become superluminal when the required particle velocity exceeds the speed of light, effectively inhibiting any upstream particle flow. In order to determine whether such shocks can accelerate particles, we use the particle-in-cell (PIC) method to determine what fraction of particles gets reflected initially at the shock. We then use this as input for a new simulation that combines the PIC method with grid-based magnetohydrodynamics to follow the acceleration (if any) of the particles over a larger time-period in a two-dimensional grid. We find that quasi-perpendicular, relativistic shocks are capable of accelerating particles through the DSA process, provided that the shock has a sufficiently high Alfvenic Mach number.