Search for Synchrotron Pair Echo Emission Following KM3-230213A

Abstract The KM3NeT Collaboration has recently reported the detection of an extraordinary ultrahigh-energy (HE) neutrino event with an energy of 220 PeV. UHE neutrinos and gamma rays are coproduced in UHE cosmic-ray interactions. If a UHE neutrino was produced within the large-scale structure around the source where it was accelerated, gamma-ray emission may be expected via the synchrotron pair echo mechanism. Here, we develop the synchrotron pair echo model in the specific context of the KM3NeT neutrino. Motivated by the fact that the synchrotron pair echo signal is expected to peak in the GeV–TeV band, and that the signal may appear as a dim, transient source, we investigate the data collected by the Large Area Telescope (LAT) on board the Fermi Gamma-ray Space Telescope for transient and subthreshold gamma-ray sources in the vicinity of the KM3NeT neutrino. We find three subthreshold sources with TS ≳ 16 within 3 . ° 5 of the neutrino event not included in any existing Fermi-LAT catalogs, but we note that none of the identified subthreshold sources seem to be compelling candidates for synchrotron pair echo emission.