The Propagation of Cosmic Rays from the Galactic Wind Termination Shock: Back to the Galaxy?

TL;DR

This study investigates whether cosmic rays accelerated at the Galactic wind termination shock can significantly contribute to the observed cosmic ray flux in the knee region, using detailed propagation simulations.

Contribution

It provides the first detailed simulation of cosmic ray propagation from the Galactic wind termination shock, assessing its potential contribution to the knee region flux.

Findings

Galactic wind termination shocks can significantly contribute to cosmic rays in the knee region.

Simulated neutrino fluxes from this source are below current IceCube and KM3NeT limits.

The model supports the shock as a plausible cosmic ray source at high energies.

Abstract

Although several theories for the origin of cosmic rays in the region between the spectral `knee' and `ankle' exist, this problem is still unsolved. A variety of observations suggest that the transition from Galactic to extragalactic sources occurs in this energy range. In this work we examine whether a Galactic wind which eventually forms a termination shock far outside the Galactic plane can contribute as a possible source to the observed flux in the region of interest. Previous work by Bustard et al. (2017) estimated that particles can be accelerated up to energies above the `knee' up to $R_\mathrm{max} = 10^{16}$ eV for parameters drawn from a model of a Milky Way wind (Everett et al. 2017). A remaining question is whether the accelerated cosmic rays can propagate back into the Galaxy. To answer this crucial question, we simulate the propagation of the cosmic rays using the low energy extension of the CRPropa framework, based on the solution of the transport equation via stochastic differential equations. The setup includes all relevant processes, including three-dimensional anisotropic spatial diffusion, advection, and corresponding adiabatic cooling. We find that, assuming realistic parameters for the shock evolution, a possible Galactic termination shock can contribute significantly to the energy budget in the `knee' region and above. We estimate the resulting produced neutrino fluxes and find them to be below measurements from IceCube and limits by KM3NeT.