Active Galactic Nuclei as high-energy neutrino sources

Abstract

Identifying the sources of the high-energy astrophysical neutrinos has been one of the main topics in astrophysics since the first observation of high-energy neutrinos by the IceCube Neutrino Observatory. Active Galactic Nuclei (AGN) are sources of high-energy gamma-rays and are considered to be promising candidates to be sources of high-energy neutrinos and ultra-high energy cosmic rays as well. However, several studies suggest that the neutrino emission from the $γ$-ray blazar population only accounts for a small fraction of the total astrophysical neutrino flux. We present and discuss recent results on the search for correlations between astrophysical neutrinos and both gamma-ray and radio bright AGN. The IceCube Collaboration has reported high-energy neutrino events that may come from both the radio-loud AGN TXS 0506+056 and the radio-quiet AGN NGC 1068. Other cases of possible associations between high-energy neutrino events and individual blazars were claimed with controversial results. We discuss the properties of these sources together with the different neutrino production mechanisms proposed for those sources. Finally, we outline future prospects in the field, focusing on remaining open questions, the development of upcoming neutrino facilities, and the evolving multi-frequency landscape within the multi-messenger era.