Time-dependent particle acceleration in supernova remnants in different environments

TL;DR

This study uses a Monte Carlo and MHD simulation to explore how different environments affect particle acceleration in supernova remnants, revealing significant differences from steady-state models and highlighting the role of the surrounding medium and reverse shock in cosmic ray acceleration.

Contribution

It introduces a time-dependent simulation approach combining Monte Carlo diffusion with MHD evolution to analyze particle acceleration in supernova remnants across various environments.

Findings

Density profile significantly influences maximum particle energy.

Time-dependent models differ from steady-state in spectral shape and cutoff.

Reverse shocks can re-accelerate cosmic rays if magnetic fields are strong.

Abstract

We simulate time-dependent particle acceleration in the blast wave of a young supernova remnant (SNR), using a Monte Carlo approach for the diffusion and acceleration of the particles, coupled to an MHD code. We calculate the distribution function of the cosmic rays concurrently with the hydrodynamic evolution of the SNR, and compare the results with those obtained using simple steady-state models. The surrounding medium into which the supernova remnant evolves turns out to be of great influence on the maximum energy to which particles are accelerated. In particular, a shock going through a $\rho \propto r^{-2}$ density profile causes acceleration to typically much higher energies than a shock going through a medium with a homogeneous density profile. We find systematic differences between steady-state analytical models and our time-dependent calculation in terms of spectral slope, maximum energy, and the shape of the cut-off of the particle spectrum at the highest energies. We also find that, provided that the magnetic field at the reverse shock is sufficiently strong to confine particles, cosmic rays can be easily re-accelerated at the reverse shock.