A signature of anisotropic cosmic-ray transport in the gamma-ray sky

TL;DR

This paper investigates how anisotropic cosmic-ray diffusion along magnetic fields explains the observed spectral hardening of protons in the inner Galaxy, using numerical modeling with the DRAGON code.

Contribution

It introduces a model of anisotropic cosmic-ray diffusion aligned with Galactic magnetic fields to explain gamma-ray observations, advancing understanding of cosmic-ray transport mechanisms.

Findings

Anisotropic diffusion can account for the proton spectral hardening in the inner Galaxy.

Numerical models reproduce the gamma-ray inferred proton slopes.

The proposed model depends on key free parameters related to magnetic field structure.

Abstract

A crucial process in Galactic cosmic-ray (CR) transport is the spatial diffusion due to the interaction with the interstellar turbulent magnetic field. Usually, CR diffusion is assumed to be uniform and isotropic all across the Galaxy. However, this picture is clearly inaccurate: Several data-driven and theoretical arguments, as well as dedicated numerical simulations, show that diffusion exhibits highly anisotropic properties with respect to the direction of a background (ordered) magnetic field (i.e., parallel or perpendicular to it). In this paper we focus on a recently discovered anomaly in the hadronic CR spectrum inferred by the Fermi-LAT gamma-ray data at different positions in the Galaxy, i.e. the progressive hardening of the proton slope at low Galactocentric radii. We propose the idea that this feature can be interpreted as a signature of anisotropic diffusion in the complex Galactic magnetic field: In particular, the harder slope in the inner Galaxy is due, in our scenario, to the parallel diffusive escape along the poloidal component of the large-scale, regular, magnetic field. We implement this idea in a numerical framework, based on the DRAGON code, and perform detailed numerical tests on the accuracy of our setup. We discuss how the effect proposed depends on the relevant free parameters involved. Based on low-energy extrapolation of the few focused numerical simulations aimed at determining the scalings of the anisotropic diffusion coefficients, we finally present a set of plausible models that reproduce the behavior of the CR proton slopes inferred by gamma-ray data.