The Sun and the supernova 1987A produce astrophysical neutrinos, but with much lower energies than the highest-energy neutrino observed to date. Of the many candidate sources of neutrinos in the giga- to tera-electronvolt range, blazars — active galactic nuclei with relativistic jets — are the most promising. Although the direction of the 290-TeV neutrino event reported by the IceCube Collaboration in 2017 coincided with a flaring blazar, the neutrino’s origin is not yet fully understood — motivating further searches.
Kaito Hagiwara and colleagues have searched for neutrinos from the direction of that very same blazar. With data collected over two decades by the Super-Kamiokande detector — a water Cerenkov detector located a thousand metres underground in Japan — the team looked for an excess of neutrinos over the atmospheric background. They found no excess of neutrinos in the direction of the blazar over the whole period as well as localized over shorter time spans. The quest continues.