A Hadronic-Leptonic Model for the Fermi Bubbles: Cosmic-Rays in the Galactic Halo and Radio Emission

TL;DR

This paper models the Fermi bubbles' gamma-ray and radio emissions using a hadronic cosmic-ray interaction framework, predicting shock front locations and explaining observed emissions and temperature profiles.

Contribution

It introduces a detailed hadronic model for the Fermi bubbles, including cosmic-ray acceleration, interaction, and re-acceleration effects, aligning with observations.

Findings

Gamma-ray spectra and profiles match observations.

Shock front is ahead of the gamma-ray boundary.

Secondary electrons alone cannot explain radio emission.

Abstract

We investigate non-thermal emission from the Fermi bubbles on a hadronic model. Cosmic-ray (CR) protons are accelerated at the forward shock of the bubbles. They interact with the background gas in the Galactic halo and create $\pi^0$-decay gamma-rays and secondary electrons through proton-proton interaction. We follow the evolution of the CR protons and electrons by calculating their distribution functions. We find that the spectrum and the intensity profile of $\pi^0$-decay gamma-rays are consistent with observations. We predict that the shock front is located far ahead of the gamma-ray boundary of the Fermi bubbles. This naturally explains the fact that a clear temperature jump of thermal gas was not discovered at the gamma-ray boundary in recent Suzaku observations. We also consider re-acceleration of the background CRs in the Galactic halo at the shock front. We find that it can significantly affect the gamma-rays from the Fermi bubbles, unless the density of the background CRs is $\lesssim 10$\% of that in the Galactic disk. We indicate that secondary electrons alone cannot produce the observed radio emission from the Fermi bubbles. However, the radio emission from the outermost region of the bubbles can be explained, if electrons are directly accelerated at the shock front with an efficiency of $\sim 0.1$\% of that of protons.