Two emission mechanisms in the Fermi Bubbles: A possible signal of annihilating dark matter

We study the variation of the spectrum of the Fermi Bubbles with Galactic latitude. Far from the Galactic plane (vertical bar b greater than or similar to 30 degrees), the observed gamma-ray emission is nearly invariant with latitude, and is consistent with arising from inverse Compton scattering of the interstellar radiation field by cosmic-ray electrons with an approximately power-law spectrum. The same electrons in the presence of microgauss-scale magnetic fields can also generate the the observed microwave haze. At lower latitudes (vertical bar b vertical bar less than or similar to 20 degrees), in contrast, the spectrum of the emission correlated with the Bubbles possesses a pronounced spectral feature peaking at similar to 1-4 GeV (in E(2)dN/dE) which cannot be generated by any realistic spectrum of electrons. Instead, we conclude that a second (non-inverse-Compton) emission mechanism must be responsible for the bulk of the low-energy, low-latitude emission. This second component is spectrally similar to the excess GeV emission previously reported from the Galactic Center (GC), and also appears spatially consistent with a luminosity per volume falling approximately as r(-2.4), where r is the distance from the GC. Consequently, we argue that the spectral feature visible in the low-latitude Bubbles is most likely the extended counterpart of the GC excess, now detected out to at least similar to 2-3 kpc from the GC. The spectrum and angular distribution of the signal is broadly consistent with that predicted from similar to 10 GeV dark matter particles annihilating to leptons, or from similar to 50 GeV dark matter particles annihilating to quarks, following a distribution similar to, but slightly steeper than, the canonical Navarro-Frenk-White (NFW) profile. We also consider millisecond pulsars as a possible astrophysical explanation for the signal, as observed millisecond pulsars possess a spectral cutoff at approximately the required energy. Any such scenario would require a large population of unresolved millisecond pulsars extending at least 2-3 kpc from the GC. (C) 2013 The Authors. Published by Elsevier B.V.