Hidden Cores of Active Galactic Nuclei as the Origin of Medium-Energy
  Neutrinos: Critical Tests with the MeV Gamma-Ray Connection
  Mysteries about the origin of high-energy cosmic neutrinos have deepened by
the recent IceCube measurement of a large diffuse flux in the 10-100 TeV range.
Based on the standard disk-corona picture of active galactic nuclei (AGN), we
present a phenomenological model enabling us to systematically calculate the
spectral sequence of multimessenger emission from the AGN coronae. We show that
protons in the coronal plasma can be stochastically accelerated up to PeV
energies by plasma turbulence, and find that the model explains the large
diffuse flux of medium-energy neutrinos if the cosmic rays carry only a few
percent of the thermal energy. We find that the Bethe-Heitler process plays a
crucial role in connecting these neutrinos and cascaded MeV gamma rays, and
point out that the gamma-ray flux can even be enhanced by the reacceleration of
secondary pairs. Critical tests of the model are given by its prediction that a
significant fraction of the MeV gamma-ray background correlates with about 10
TeV neutrinos, and nearby Seyfert galaxies including NGC 1068 are promising
targets for IceCube, KM3Net, IceCube-Gen2, and future MeV gamma-ray telescopes.
