Plasma Wave-particle Acceleration as the Origin of the Galactic Gamma-Ray Bubbles

Abstract The “Fermi bubbles” are two giant gamma-ray emitting configurations in the form of bubbles that extend above and below the Galactic plane. They are characterized by a smooth surface, sharp edges, and an almost flat intensity distribution. They were discovered in 2010 by the Fermi telescope and detected in the energy range 1–100 GeV. We propose a novel theoretical model to explain the process involved in the “Fermi bubbles” phenomenon. The model is based on the “in situ” generation of an energetic electron population powered by unstable electromagnetic waves triggered by a nonrelativistic outflow of matter escaping from the Galactic center region. A hadronic outflow crossing the magnetic field in the Galactic halo of low-density plasma can induce an electromagnetic instability in the “lower hybrid” frequency range (between the ion and electron cyclotron frequencies). These lower hybrid waves propagate along the magnetic field lines in the halo and can accelerate, via resonant Landau damping, the thermal electrons up to energies on the order of hundreds of GeV. This energetic electron population, localized in a thin boundary layer of the halo, radiates by synchrotron emission and by inverse Compton scattering. An evaluation of the radiation emission and a comparison with spectral data is presented. This model successfully explains in a natural way the GeV spectral properties of the “Fermi bubbles” as well as their microwave emission.