  The IceCube neutrino observatory has detected two cascade events with
energies near 1 PeV. Without invoking new physics, we analyze the source of
these neutrinos. We show that atmospheric conventional neutrinos and cosmogenic
neutrinos (those produced in the propagation of ultra-high- energy cosmic rays)
are strongly disfavored. For atmospheric prompt neutrinos or a diffuse
background of neutrinos produced in astrophysical objects, the situation is
less clear. We show that there are tensions with observed data, but that the
details depend on the least-known aspects of the IceCube analysis. Very likely,
prompt neutrinos are disfavored and astrophysical neutrinos are plausible. We
demonstrate that the fastest way to reveal the origin of the observed PeV
neutrinos is to search for neutrino cascades in the range below 1 PeV, for
which dedicated analyses with high sensitivity have yet to appear, and where
many more events could be found.
