Cosmic-ray propagation under consideration of a spatially resolved source distribution

TL;DR

This paper develops a 3D cosmic-ray source distribution model based on observations and simulations, enabling more realistic Galactic cosmic-ray propagation and gamma-ray emission predictions beyond traditional symmetric models.

Contribution

It introduces a hybrid, observation-based 3D source distribution model for cosmic-ray propagation simulations, improving realism over previous symmetric approaches.

Findings

Models show global consistency with previous simulations.

Local fluctuations observed in spiral arms.

Enhanced realism in gamma-ray sky modeling.

Abstract

Cosmic rays (CRs) are an integral component of the interstellar medium, producing broadband emission while interacting with other Galactic matter components like the interstellar gas or magnetic fields. In addition to observations, numerical simulations of CR propagation through the Galaxy help to increase the level of understanding of Galactic CR transport and diffuse $\gamma$-ray emission as seen by different experiments. Up to now, the standard approach at modelling source distributions used as input for such transport simulations often rely on radial symmetry and analytical functions rather than individual, observation-based sources. We aim at a redefinition of existing CR source distributions by combining sources observed with the H.E.S.S. experiment and simulated random sources, which follow the matter density in the Milky Way. As a result, H.E.S.S.-inspired Galactic CR source distributions are inferred. We use the PICARD code to perform 3D-simulations of nuclei and electrons in CR propagation using our hybrid source distribution models. Furthermore, also gamma-ray maps and spectra, simulated with the redefined source models, are evaluated in different regions in the Galaxy and compared with each other to determine the statistical scatter of the underlying distributions. We find global consistency between our models and in comparison to previous simulations, with only some localised fluctuations, e.g. in the spiral arms. This implementation of a three-dimensional source model based on observations and simulations enables a new quality of propagation modelling. It offers possibilities for more realistic CR transport scenarios beyond radial symmetry and delivers meaningful results in both the arm and interarm regions of the Galaxy. This gives a more realistic picture of the Galactic $\gamma$-ray sky by including structures from the source model and not just the gas distributions.