Testing the influence of anisotropic CR transport and the Galactic magnetic field structure on the all-sky gamma-ray emission

TL;DR

This study investigates how anisotropic cosmic ray diffusion influenced by the Galactic magnetic field affects the all-sky gamma-ray emission, revealing significant impacts on spatial distribution and spectral characteristics.

Contribution

It introduces a detailed modeling of anisotropic cosmic ray transport in the Milky Way and assesses its effects on gamma-ray emission using the CRPropa code.

Findings

Anisotropic diffusion significantly alters gamma-ray flux distribution.

Magnetic field geometry strongly influences gamma-ray spectral features.

Diffusion anisotropy impacts cosmic ray residence time and energy spectra.

Abstract

The spatial diffusion of energetic particles in a magnetic field composed of a large-scale background and a small-scale turbulent component should be expected to be anisotropic. While such anisotropic diffusion has been known for quite a while in first-principle plasma physics and while it is required for an understanding of the transport of cosmic rays in the heliosphere or close to supernova remnants, only in recent years it has also become of particular interest for the modeling of Galactic cosmic ray (GCR) transport in the Milky Way in the context of their residence time and their (local) energy spectra. Also, the large-scale spatial distribution of GCRs is shaped by an anisotropic diffusion in the Galactic magnetic field, which should directly affect both the diffuse gamma-ray and the neutrino emission. We solve the anisotropic diffusive transport of GCRs in the Milky Way using the publicly available transport code CRPropa. The anisotropy of the diffusion is characterized by the ratio between the diffusion coefficient perpendicular and parallel to the local magnetic field $\epsilon = D_\perp / D_\parallel$, where we test different values reaching from nearly parallel transport ($\epsilon = 10^{-3}$) to more isotropic diffusion ($\epsilon = 10^{-1}$). From the three dimensional distribution of GCRs in the Milky Way we calculate the all-sky gamma-ray emission, using the line-of-sight integration framework HERMES. Finally, we demonstrate the impact of the anisotropy in the diffusion on the spatial distribution of the gamma-ray flux and its spectral energy distribution. It shows strong influences by the anisotropy of the diffusion and the magnetic field geometry.