Galactic PeV neutrinos from dark matter annihilation
  The IceCube Neutrino Observatory has observed highly energetic neutrinos in
excess of the expected atmospheric neutrino background. It is intriguing to
consider the possibility that such events are probing physics beyond the
standard model. In this context, $\mathcal{O}$(PeV) dark matter particles
decaying to neutrinos have been considered while dark matter annihilation has
been dismissed invoking the unitarity bound as a limiting factor. However, the
latter claim was done ignoring the contribution from dark matter substructure,
which for PeV Cold Dark Matter would extend down to a free streaming mass of
$\mathcal{O}$($10^{-18}$M$_\odot$). Since the unitarity bound is less stringent
at low velocities, ($\sigma_{\rm ann}$v)$\leq4\pi/m_\chi^2v$, then, it is
possible that these cold and dense subhalos would contribute dominantly to a
dark-matter-induced neutrino flux and easily account for the events observed by
IceCube. A Sommerfeld-enhanced dark matter model can naturally support such
scenario. Interestingly, the spatial distribution of the events shows features
that would be expected in a dark matter interpretation. Although not
conclusive, 9 of the 37 events appear to be clustered around a region near the
Galactic Center while 6 others spatially coincide, within the reported angular
errors, with 5 of 26 Milky Way satellites. However, a simple estimate of the
probability of the latter occurring by chance is $\sim35\%$. More events are
needed to statistically test this hypothesis. PeV dark matter particles are
massive enough that their abundance as standard thermal relics would overclose
the Universe. This issue can be solved in alternative scenarios, for instance
if the decay of new massive unstable particles generates significant entropy
reheating the Universe to a slightly lower temperature than the freeze-out
temperature, $T_{\rm RH} \lesssim T_{\rm f}\sim4\times10^4$~GeV.
