Numerical Modeling of Time Dependent Diffusive Shock Acceleration

TL;DR

This paper develops a numerical model for time-dependent diffusive shock acceleration, incorporating importance sampling and adaptive time steps, to study cosmic ray re-acceleration at shocks like the Galactic Wind Termination Shock.

Contribution

It introduces a modified CRPropa3.2 framework with importance sampling and adaptive time steps for accurate, efficient modeling of time-dependent DSA, including application to the GWTS.

Findings

Injected spectra influence the shock spectrum shape.

The model accurately reproduces known solutions for shock spectra.

Application to GWTS shows realistic spectral evolution.

Abstract

Motivated by cosmic ray (CR) re-acceleration at a potential Galactic Wind Termination Shock (GWTS), we present a numerical model for time-dependent Diffusive Shock Acceleration (DSA). We use the stochastic differential equation solver (DiffusionSDE) of the cosmic ray propagation framework CRPropa3.2 with two modifications: An importance sampling module is introduced to improve statistics at high energies in order to keep the simulation time short. An adaptive time step is implemented in the DiffusionSDE module. This ensures to efficiently meet constraints on the time and diffusion step, which is crucial to obtain the correct shock spectra. The time evolution of the spectrum at a one-dimensional planar shock is verified against the solution obtained by the grid-based solver VLUGR3 for both energy-independent and energy-dependent diffusion. We show that the injection of pre-accelerated particles can lead to a broken power law spectrum in momentum if the incoming spectrum of CRs is harder than the re-accelerated spectrum. If the injected spectrum is steeper, the shock spectrum dominates at all energies. We finally apply the developed model to the GWTS by considering a spherically symmetric shock, a spiral Galactic magnetic field, and anisotropic diffusion. The time-dependent spectrum at the shock is modeled as a basis for further studies.