Constraining the Sommerfeld enhancement with Cherenkov telescope
  observations of dwarf galaxies
  The presence of dark matter in the halo of our galaxy could be revealed
through indirect detection of annihilation products. Dark matter annihilation
is one of the possible interpretations of the recent measured excesses in
positron and electron fluxes, once boost factors of the order of 10^3 or more
are taken into account. Such boost factors are actually achievable through the
velocity-dependent Sommerfeld enhancement of the annihilation cross-section.
Here we study the expected gamma-ray flux from two local dwarf galaxies for
which air Cerenkov measurements are available, namely Draco and Sagittarius. We
use velocity dispersion measurements to model the dark matter halos of the
dwarfs, and the results of numerical simulations to model the presence of an
associated population of subhalos. We incorporate the Sommerfeld enhancement of
the annihilation cross-section. We compare our predictions with observations of
Draco and Sagittarius performed by MAGIC and HESS, respectively. We also
compare our results with the sensitivities of Fermi and of the future Cherenkov
Telescope Array. We find that the boost factor due to the Sommerfeld
enhancement is already constrained by the MAGIC and HESS data, with
enhancements greater than 5 x 10^4 being excluded. While Fermi will not be able
to detect gamma-rays from the dwarf galaxies s even with the most optimistic
Sommerfeld effect, we show that the Cherenkov Telescope Array will be able to
test enhancements greater than 1.5 x 10^3.
