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

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.

ApJ Letters accepted, major revision after referee's comments

References in corpus (10)

• Giant Gamma-ray Bubbles from Fermi-LAT: AGN Activity or Bipolar Galactic Wind?
• Cosmic-Ray Proton and Helium Spectra from the First CREAM Flight
• Global cosmic-ray related luminosity and energy budget of the Milky Way
• The Fermi Bubbles: Giant, Multi-Billion-Year-Old Reservoirs of Galactic Center Cosmic Rays
• Extended Anomalous Foreground Emission in the WMAP 3-Year Data
• Giant Magnetized Outflows from the Centre of the Milky Way
• The Fermi Bubbles: Supersonic AGN Jets with Anisotropic Cosmic Ray Diffusion
• Planck Intermediate Results. IX. Detection of the Galactic haze with Planck
• Magnetic substructure in the northern Fermi Bubble revealed by polarized WMAP emission
• On the contribution of nearby sources to the observed cosmic-ray nuclei

Cited by in corpus (17)

• A Unified Model of the Fermi Bubbles, Microwave Haze, and Polarized Radio Lobes: Reverse Shocks in the Galactic Center's Giant Outflows
• Nuclear Outflow of the Milky Way: Studying the Kinematics and Spatial Extent of the Northern Fermi Bubble
• The Accretion Wind Model of the Fermi Bubbles (II): Radiation
• A Search for Neutrino Emission from the Fermi Bubbles with the ANTARES Telescope
• A Hadronic-Leptonic Model for the Fermi Bubbles: Cosmic-Rays in the Galactic Halo and Radio Emission
• Multi-wavelength Emission from the Fermi Bubble II. Secondary Electrons and the Hadronic Model of the Bubble
• A multi-messenger study of the Fermi Bubbles: very high energy gamma rays and neutrinos
• Fermi bubble edges: spectrum and diffusion function
• A Latitude-Dependent Analysis of the Leptonic Hypothesis for the Fermi Bubbles
• The Fermi bubbles from stochastic acceleration of electrons in a Galactic outflow
• Fermi bubbles: high latitude X-ray supersonic shell
• Metal Enrichment in the Fermi Bubbles as a Probe of Their Origin
• Emission from hadronic and leptonic processes in galactic jet-driven bubbles
• Recent results from the ANTARES neutrino telescope
• A possible origin of gamma rays from the Fermi Bubbles
• The Fermi bubbles as a Superbubble
• Multi-wavelength emission from leptonic processes in ageing galaxy bubbles