On The Origin of IceCube's PeV Neutrinos
  The IceCube collaboration has recently reported the observation of two events
with energies in excess of 1 PeV. While an atmospheric origin of these events
cannot be ruled out at this time, this pair of showers may potentially
represent the first observation of high-energy astrophysical neutrinos. In this
paper, we argue that if these events are neutrino-induced, then the neutrinos
are very likely to have been produced via photo-meson interactions taking place
in the same class of astrophysical objects that are responsible for the
acceleration of the $\sim$$10^{17}$ eV cosmic ray spectrum. Among the proposed
sources of such cosmic rays, gamma-ray bursts stand out as particularly capable
of generating PeV neutrinos at the level implied by IceCube's two events. In
contrast, the radiation fields in typical active galactic nuclei models are
likely dominated by lower energy (UV) photons, and thus feature higher energy
thresholds for pion production, leading to neutrino spectra which peak at EeV
rather than PeV energies (models with significant densities of x-ray emission,
however, could evade this problem). Cosmogenic neutrinos generated from the
propagation of ultra-high energy cosmic rays similarly peak at energies that
are much higher than those of the events reported by IceCube.
