On-Shell Mediators and Top-Charm Dark Matter Models for the Fermi-LAT
  Galactic Center Excess
  An excess in gamma-rays from the galactic center observed by the Fermi Large
Area Telescope has been proposed as a possible signal of dark matter
annihilation. Recently, the Fermi collaboration showed that systematic errors
broaden the range of spectral shapes for this excess. We demonstrate fits to
this range for (1) flavor-violating annihilations to top-charm pairs and (2)
annihilations to on-shell bosonic mediators which decay to Standard Model
quarks in a boosted frame. Annihilation of 40 - 100 GeV DM to pairs of spin-1
mediators provide a good fit to the Fermi-LAT spectrum with a normalization
consistent with a thermal relic. Top-charm modes and annihilation to three
pseudoscalar mediators can fit the spectral shape but typically require
non-thermal annihilation cross sections.
