Revisiting AGN as the Source of IceCube's Diffuse Neutrino Flux
  The origin of the astrophysical neutrino flux reported by the IceCube
Collaboration remains an open question. In this study, we use three years of
publicly available IceCube data to search for evidence of neutrino emission
from the blazars and non-blazar Active Galactic Nuclei (AGN) contained the
Fermi 4LAC catalog. We find no evidence that these sources produce high-energy
neutrinos, and conclude that blazars can produce no more than 15% of IceCube's
observed flux. The constraint we derive on the contribution from non-blazar
AGN, which are less luminous and more numerous than blazars, is significantly
less restrictive, and it remains possible that this class of sources could
produce the entirety of the diffuse neutrino flux observed by IceCube. We
anticipate that it will become possible to definitively test such scenarios as
IceCube accumulates and releases more data, and as gamma-ray catalogs of AGN
become increasingly complete. We also comment on starburst and other
starforming galaxies, and conclude that these sources could contribute
substantially to the signal observed by IceCube, in particular at the lowest
detected energies.
