High-Energy Extragalactic Neutrino Astrophysics
  The detection of an astrophysical flux of neutrinos in the TeV-PeV energy
range by the IceCube observatory has opened new possibilities for the study of
extreme cosmic accelerators. The apparent isotropy of the neutrino arrival
directions favors an extragalactic origin for this flux, potentially created by
a large population of distant sources. Recent evidence for the detection of
neutrino emission from extragalactic sources include the active galaxies TXS
0506+056 and NGC 1068. We here review the current status of the search for the
sources of the high-energy neutrino flux, concentrating on its extragalactic
contribution. We discuss the implications of these observations for
multimessenger studies of cosmic sources and present an outlook for how
additional observations by current and future instruments will help address
fundamental questions in the emerging field of high-energy neutrino astronomy.
