Cosmic rays prove that our Universe hosts elusive astrophysical "monsters" capable of continuously and efficiently accelerating particles at extreme energies. High-energy photons and neutrinos are the key to ultimately decipher the mystery of cosmic rays. In 2017, the discovery of neutrino emission from the direction of the gamma-ray flaring blazar TXS 0506+056 has put forward gamma-ray blazars as promising neutrino point-sources, hence cosmic-ray accelerators. However, to date there is neither a consistent picture for the physical mechanism nor a theoretical framework capable of convincingly explain the full set of multi-messenger observations. I will present initial encouraging steps in this multimessenger (electromagnetic and neutrino) field and discuss the rewarding potential of future studies.
Flavio Seno