Fermi bubble $\gamma$-rays as a result of diffusive injection of Galactic cosmic rays

TL;DR

This paper proposes that the Fermi bubbles' gamma-ray emission results from diffusive injection of Galactic cosmic-ray protons, explaining their spectrum and luminosity without additional particle production processes.

Contribution

It introduces a novel model where slow diffusion and adiabatic losses inside the bubbles account for the gamma-ray observations, differing from previous explanations.

Findings

Model reproduces observed intensity profile

Explains spectrum and luminosity of Fermi bubbles

No need for extra particle production processes

Abstract

Recently, the {\it{Fermi}} space telescope has discovered two large $\gamma$-ray emission regions, the so-called "Fermi bubbles", that extend up to $\sim 50^\circ$ above and below the Galactic center. The $\gamma$-ray emission from the bubbles are found to follow a hard spectrum with no significant spatial variation in intensity and spectral shape. The origin of the emission is still not clearly understood. Suggested explanations include injection of cosmic-ray nuclei from the Galactic center by high-speed Galactic winds, electron acceleration by multiple shocks and stochastic electron acceleration inside the bubbles. In this letter, it is proposed that the $\gamma$-rays can be the result of diffusive injection of Galactic cosmic-ray protons during their propagation through the Galaxy. Considering that the bubbles are slowly expanding, and cosmic rays undergo much slower diffusion inside the bubbles than in the averaged Galaxy and at the same time suffer losses due to adiabatic expansion and inelastic collisions with the bubble plasma, this model can explain the observed intensity profile, the emission spectrum and the measured luminosity without invoking any additional particle production processes unlike other existing models.