The role of magnetic fields in shaping $\gamma$-ray emission from the Fermi bubbles

TL;DR

This paper explores how magnetic fields influence cosmic ray diffusion and shape the gamma-ray emission features of the Fermi bubbles, providing a magnetohydrodynamic model that explains their sharp edges.

Contribution

It introduces a magnetohydrodynamic Galactic breeze model incorporating magnetic field effects and anisotropic cosmic ray diffusion to explain Fermi bubble gamma-ray morphology.

Findings

Magnetic fields cause anisotropic cosmic ray diffusion shaping gamma-ray emission.

The model produces Fermi bubble edges consistent with observations.

Magnetic effects are crucial for understanding Fermi bubble structure.

Abstract

Despite their discovery fifteen years ago, the nature and origin of the Fermi bubbles remain unclear. We here investigate the effect a magnetic field can have on a subsonic breeze outflow emanating from the Galactic centre region. The presence of this magnetic field allows anisotropic diffusion of cosmic rays within the outflow, shaping the resultant cosmic ray distribution obtained out at large distances within the Galactic halo. We show that our magnetohydrodynamic Galactic breeze model, in combination with an opening angle for the injection of cosmic rays, leads to $\gamma$-ray emission from the Fermi bubble region with relatively sharp edges.