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 (|b| > 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 (b < 20 degrees), in
contrast, the spectrum of the emission correlated with the Bubbles possesses a
pronounced spectral feature peaking at 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. We argue that the spectral feature visible in the low-latitude Bubbles is
the extended counterpart of the GC excess, now detected out to at least 2-3 kpc
from the GC. The spectrum and angular distribution of the signal is consistent
with that predicted from ~10 GeV dark matter particles annihilating to leptons,
or from ~50 GeV dark matter particles annihilating to quarks, following a
distribution similar to 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.
