Abstract
The origin of high-energy cosmic neutrinos is one of the biggest mysteries in
astroparticle physics. The fact that diffuse intensities of high-energy
neutrinos, ultrahigh-energy cosmic rays, and GeV-TeV gamma rays are all
comparable suggests that these messengers are physically connected. The IceCube
data above 100 TeV energies can be naturally explained by cosmic-ray reservoir
models. In particular, starburst galaxies and galaxy clusters/groups serve as
natural storage rooms of cosmic rays, and it has been theoretically predicted
that these sources are promising sites of high-energy neutrinos and gamma rays
that are produced via inelastic pp interactions. Indeed, the predictions made
before the discovery of IceCube neutrinos are consistent with the current
high-energy neutrino data measured in IceCube, and that they could give a
grand-unified view of sub-PeV neutrinos, sub-TeV gamma rays, and
ultrahigh-energy cosmic rays. These unified models have strong prediction
powers, which can be tested by next-generation neutrino detectors such as
IceCube-Gen2 as well as gamma-ray telescopes such as CTA. The recent
observations have also shown that the 10-100 TeV diffuse neutrino flux is
higher than that at PeV energies, which suggests that they come from a
different class of neutrino sources. The detailed comparison with the diffuse
isotropic gamma-ray background measured by Fermi has revealed that these
medium-energy neutrinos are likely to come from hidden cosmic-ray accelerators,
from which neutrinos can escape while GeV-TeV gamma rays are attenuated. The
candidate source classes are choked gamma-ray burst jets and active galactic
nuclei (AGN) cores. In particular, the AGN corona model predicts a unique
connection between 10-100 TeV neutrinos and MeV gamma rays, which can be
robustly tested with future MeV gamma-ray missions such as AMEGO.
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