The KM3NeT Collaboration has recently reported the detection of a neutrino event with energy in excess of 100 PeV. This detection is in $2.5-3\sigma$ statistical tension with the upper limit on the neutrino flux at this energy imposed by IceCube and the Pierre Auger Observatory, if the event is considered part of the diffuse all-sky neutrino flux. We explore an alternative possibility that the event originates from a flare of a rare transient source. We show that the data of KM3NeT, IceCube and the Pierre Auger Observatory are consistent with the possibility of a source flare of duration $T\lesssim 2\mathrm{yr}$ with muon neutrino flux $F\sim 3\times {10}^{-10}(1\mathrm{yr}/T)\mathrm{erg}/({\mathrm{cm}}^2\mathrm{s})$. Constraints on the neutrino spectrum indicate that the protons responsible for the neutrino emission have a very hard spectrum in the $E_p\gtrsim {10}^{19}\mathrm{eV}$ energy range, or otherwise that the neutrinos are produced by photohadronic interactions with infrared photons. The all-sky rate of similar neutrino flaring sources is constrained to be $\mathcal{R}\lesssim 0.4/\mathrm{yr}$.

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• Cosmic ray sources
• Cosmic rays & astroparticles
• Transient & explosive astronomical phenomena