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

ApJ in press

References in corpus (6)

• Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic Wind?
• Inverse Compton Origin of the Hard X-Ray and Soft Gamma-Ray Emission from the Galactic Ridge
• Giant Magnetized Outflows from the Centre of the Milky Way
• Parameterization of the Angular Distribution of Gamma Rays Produced by p-p Interaction in Astronomical Environment
• A new estimate of the Galactic interstellar radiation field between 0.1 microns and 1000 microns
• Non-thermal emission from old supernova remnants

Cited by in corpus (6)

• The Interaction of the Fermi Bubbles with the Milky Way's Hot Gas Halo
• The Spatially Uniform Spectrum of the Fermi Bubbles: the Leptonic AGN Jet Scenario
• Simulating the Fermi Bubbles as Forward Shocks Driven by AGN Jets
• Suzaku X-ray Observations of the Fermi Bubbles: Northernmost Cap and Southeast Claw Discovered with MAXI-SSC
• Metal Enrichment in the Fermi Bubbles as a Probe of Their Origin
• A Novel Approach towards the Search for Gamma-ray Emission from the Northern Fermi Bubble with HAWC