Evidence for a hadronic origin of the Fermi Bubbles and the Galactic Excess

Abstract

The Fermi-LAT gamma-ray data revealed giant ’bubbles’ of emission above and below the Galactic plane with an energy spectrum significantly harder than seen from other directions. How the bubbles connect to the Galactic plane is unclear. Previous analyses masked the Galactic plane because of the large foreground. In this paper we use a novel spectral template fit, which allows a simultaneous determination of the foreground and the hard bubble-like emission in any direction. We find that bubble-like emission is not only found in the halo, but it is strongly present in the Galactic plane as well with a morphology close to the spatial distribution of the 1.8 MeV gamma-ray line from 26 Al, a radioactive nucleus synthesized in SNRs. In addition, the spectral shape of this hard component coincides with the predicted spectrum from cosmic rays trapped in sources (SCRs) Hence, we propose that the bubble-like emission in the plane has a hadronic origin, which arises from SCRs. The bubbles in the halo have the same energy spectrum, which suggests that they are outflows from the plane with the gamma-rays arising from hadronic interactions of protons trapped in a plasma of advected gas. Evidence for advected gas is provided by the ROSAT X-ray observations from hot gas in the bubble region. Alternatively, the protons may be accelerated in the shock wave of the outflow, thus having the same spectrum as the SCRs, which are accelerated in shock waves as well. Towards the Galactic center (GC) we observe the 1-3 GeV excess, but observe this excess in addition in all regions where there is strong 26 Al production. This excludes the dark matter annihilation interpretation. Instead, we propose that the excess is caused by a deficit of the CRs at low rigidities in the regions of dense molecular clouds, which are characterized by higher energy losses and stronger stellar winds. The 26 Al line is a tracer of such regions. If we introduce a break in the cosmic ray injection spectra in the regions of strong 26 Al production to effectively take the deficit of low rigidity CRs into account, the excess in the Galactic plane and towards the GC both disappear. We thus conclude that the excess in the Galactic center is largely an artefact from the excess in the Galactic plane. The correlation between the two originates simply from the lines-of-sight towards the GC crossing the Galactic plane.