Cascaded Gamma-Ray Emission Associated with the KM3NeT Ultrahigh-energy Event KM3-230213A

Abstract A neutrino-like event with an energy of ∼220 PeV was recently detected by the KM3NeT/ARCA telescope. If this neutrino comes from an astrophysical source or from the interaction of an ultrahigh-energy cosmic ray in the intergalactic medium, the ultrahigh-energy gamma rays that are coproduced with the neutrinos will scatter with the extragalactic background light, producing an electromagnetic cascade and resulting in emission at GeV-to-TeV energies. In this Letter, we compute the gamma-ray flux from this neutrino source considering various source distances and strengths of the intergalactic magnetic field (IGMF). We find that the associated gamma-ray emission could be observed by existing imaging air Cherenkov telescopes and air shower gamma-ray observatories, unless the strength of the IGMF is B ≳ 3 × 10 −13 G or the ultrahigh-energy gamma rays are attenuated inside of the source itself. In the latter case, this source is expected to be radio-loud.