[1010.2752] Dark Matter Annihilation in The Galactic Center As Seen by the Fermi Gamma Ray Space Telescope

Authors:  Dan Hooper, Lisa Goodenough
Abstract:  We analyze the first two years of data from the Fermi Gamma Ray Space Telescope from the direction of the inner 10 degrees around the Galactic Center with the intention of constraining, or finding evidence of, annihilating dark matter. We find that the morphology and spectrum of the emission between 1.25 and 10 degrees from the Galactic Center is well described by a the processes of decaying pions produced in cosmic ray collisions with gas, and the inverse Compton scattering of cosmic ray electrons in both the disk and bulge of the Inner Galaxy, along with gamma rays from known points sources in the region. The observed spectrum and morphology of the emission within approximately 1.25 degrees (~175 parsecs) of the Galactic Center, in contrast, cannot be accounted for by these processes or known sources. We find that an additional component of gamma ray emission is clearly present which is highly concentrated around the Galactic Center, but is not point-like in nature. The observed morphology of this component is consistent with that predicted from annihilating dark matter with a cusped (and possibly adiabatically contracted) halo distribution (density proportional to r^{-1.34 \pm 0.04}). The observed spectrum of this component, which peaks at energies between 2-4 GeV (in E^2 units), is well fit by that predicted for a 7.3-9.2 GeV dark matter particle annihilating primarily to tau leptons with a cross section in the range of 3.3 x 10^(-27) to 1.5 x 10^(-26) cm^3/s, depending on how the dark matter distribution is normalized. We discuss other possible sources for this component, but argue that they are unlikely to account for the observed emission.
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Mark Wyman
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[1010.2752] Dark Matter Annihilation in The Galactic Center

Post by Mark Wyman » October 22 2010

So, is this it? 8 GeV dark matter that annihilates into tau leptons?

Douglas Applegate
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Affiliation: Kavli Institute for Particle Astrophysics & Cosmology (KIPAC)

[1010.2752] Dark Matter Annihilation in The Galactic Center

Post by Douglas Applegate » October 22 2010

Possibly not. Though I'm not a Fermi scientist, I work near some. One comment I overheard was that a bubble from multiple supernovae could be a viable alternative.

Also, treating the Fermi point spread function as a Gaussian is apparently a big mistake – the region of interest in this paper is at the limit of Fermi's resolution, from what I understand. (One scientist, to remain unnamed for dignity reasons, suffered a fit of giggles upon hearing about the Gaussian approximation in this paper.) Finally, I got the impression that the Galprop people were somewhat uncomfortable with how their models were used.

Hopefully, the authors and the Fermi folks will have a sit-down soon and hash out any oversimplifications. Nothing like a little competition to bring out the best science!

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