Cosmic Rays in a Galactic Breeze

TL;DR

This paper explores how cosmic rays interacting with a galactic outflow can produce gamma-ray features like the Fermi bubbles and explains observed cosmic ray spectral hardening at high energies.

Contribution

It introduces a model of galactic outflow-driven cosmic ray transport that accounts for gamma-ray emission and spectral hardening, linking these phenomena to the Fermi bubbles.

Findings

Gamma-ray emission with flat surface brightness profile explained by outflow interactions.

Spectral hardening due to advective and diffusive CR propagation in the outflow.

Potential explanation for the observed CR spectral hardening at ~200 GV.

Abstract

Motivated by the discovery of the non-thermal Fermi bubble features both below and above the Galactic plane, we investigate a scenario in which these bubbles are formed through Galacto-centric outflow. Cosmic rays (CR) both diffusing and advecting within a Galactic breeze outflow, interacting with the ambient gas present, give rise to gamma-ray emission, providing an approximately flat surface brightness profile of this emission, as observed. Applying the same outflow profile further out within the disk, the resultant effects on the observable CR spectral properties are determined. A hardening in the spectra due to the competition of advective and diffusive propagation within a particular energy range is noted, even in the limiting case of equal CR diffusion coefficients in the disk and halo. It is postulated that this hardening effect may relate to the observed hardening feature in the CR spectrum at a rigidity of $\approx 200$ GV.