Scrutinizing the cosmogenic origin of the KM3-230213A event: A Multimessenger Perspective

The recent detection of the neutrino event KM3-230213A (~~220 PeV) by the KM3NeT/ARCA telescope, the most energetic ever observed, could represent the long-awaited evidence for a cosmogenic origin, arising from the interaction of an ultra-high-energy cosmic ray with background photons. Its secure confirmation would mark a major advance in high-energy astrophysics. We perform a self-consistent multimessenger transport calculation of protons and their secondary $γ$-rays and neutrinos from cosmologically evolving sources, confronting predictions with data from the Pierre Auger Observatory, IceCube, KM3NeT, and the Fermi-LAT isotropic $γ$-ray background. A steep sub-ankle proton component saturates the diffuse $γ$-ray background and is disfavoured, whereas a hard proton spectrum extending beyond 10²⁰~eV with evolution ∝ (1+z)³ reproduces KM3-230213A without violating any limits. This scenario requires a proton fraction 10% at 3× 10¹⁹~eV and excludes faster-evolving sources. Joint UHE-neutrino and $γ$-ray observations thus sharpen constraints on extragalactic cosmic-ray sources and set targets for AugerPrime and next-generation neutrino telescopes.