p-wave Annihilating Dark Matter from a Decaying Predecessor and the
  Galactic Center Excess
  Dark matter (DM) annihilations have been widely studied as a possible
explanation of excess gamma rays from the galactic center seen by Fermi/LAT.
However most such models are in conflict with constraints from dwarf
spheroidals. Motivated by this tension, we show that p-wave annihilating dark
matter can easily accommodate both sets of observations due to the lower DM
velocity dispersion in dwarf galaxies. Explaining the DM relic abundance is
then challenging. We outline a scenario in which the usual thermal abundance is
obtained through s-wave annihilations of a metastable particle, that eventually
decays into the p-wave annihilating DM of the present epoch. The couplings and
lifetime of the decaying particle are constrained by big bang nucleosynthesis,
the cosmic microwave background and direct detection, but significant regions
of parameter space are viable. A sufficiently large p-wave cross section can be
found by annihilation into light mediators, that also give rise to Sommerfeld
enhancement. A prediction of the scenario is enhanced annihilations in galaxy
clusters.
