A Three-Dimensional Tomographic Reconstruction of the Galactic Cosmic-Ray Proton Density

TL;DR

This paper presents a novel 3D map of Galactic cosmic-ray proton density derived from gamma-ray observations, enhancing understanding of CR transport and source distribution.

Contribution

The work introduces a data-driven, three-dimensional reconstruction method of CRp density using Gaussian processes and variational inference from Fermi-LAT gamma-ray data.

Findings

CRp density shows a smooth, structured distribution across the Galaxy.

Moderate enhancement of CRp density observed toward the inner Galaxy.

Inferred normalization aligns with local CR measurements by AMS-02.

Abstract

Cosmic rays (CRs) are a ubiquitous non-thermal component of the interstellar medium (ISM). A data-driven three-dimensional (3D) map of their distribution is essential for understanding CR transport and constraining the spatial distribution of their sources. In this work, we reconstructed the 3D spatial distribution of the Galactic cosmic-ray proton (CRp) density. We model the diffuse gamma-ray emission arising from inelastic hadronic interactions between CRps and interstellar gas. Using a map of dust-correlated diffuse gamma-ray emission based on ten years of Fermi-LAT observations together with a three-dimensional gas density model, we infer the spatial CRp distribution through a morphological matching approach. The logarithmic CRp density field is described by a Gaussian process defined on a spherical-times-radial grid, while both the field and its correlation structure are inferred simultaneously using Iterative Charted Refinement. The posterior distribution of the reconstructed 3D CRp density field is approximated using geometric variational inference. The reconstructed CRp density exhibits a smooth but spatially structured distribution with a limited dynamical range across the Galactic disk. We find a moderate enhancement of the CRp density toward the inner Galaxy. The inferred normalization at the Solar position is consistent with local CR measurements by the AMS-02 instrument.