Ultra-high-energy cosmic neutrinos: at energies and above

The high-energy neutrino astronomy has finally bloomed. The discovery of the first 10 15   eV (PeV or 10 6   GeV ) neutrino events by the IceCube Collaboration followed by the dedicated search for 10 12 – 10 15   eV events exhibited the evidence of an astrophysical neutrino flux. The estimated flux level is around E 2 ϕ ν e + ν μ + ν τ = 3 × 10 − 8   GeV cm − 2 s − 1 sr − 1 , consistent with the intensity expected from the energetics of extragalactic cosmic rays. The strong bound of neutrino flux in the 10 18 -eV range, which amounts to E 2 ϕ ν e + ν μ + ν τ = 1.2 × 10 − 7   GeV cm − 2 s − 1 sr − 1 at 1 × 10 19   eV provided by IceCube has reached the flux region predicted for some ultra-high-energy cosmic-ray source class candidates, leading to astrophysically meaningful constraints on the origin of cosmic rays. It disfavors strong cosmological evolution of the highest energy cosmic ray sources such as the Fanaroff–Riley type-II class of radio Galaxies. We live in an era when neutrinos provide clues to resolve long-standing mystery of ultra-high-energy cosmic rays.