Embedded deep below the Antarctic ice sheet, near the Amundsen–Scott South Pole Station, is a 1 km3 neutrino detector, the IceCube Neutrino Observatory. When a high-energy neutrino collides with an atomic nucleus in or near the detector, the interaction creates a charged lepton that moves through the ice at superluminal speeds. Specialized sensors record the Cherenkov radiation emitted by the particle and provide the data needed to reconstruct the particle track and estimate the direction of the original neutrino.
The vast majority of neutrinos observed by IceCube are created in Earth’s atmosphere from interactions of high-energy cosmic rays. Those neutrinos typically have energies near or below 100 TeV. In 2013, however, IceCube reported an apparently isotropic flux of neutrinos whose energy was so high—up to several PeV—as to be inconsistent with the usual atmospheric mechanisms. More likely sources were extragalactic ones such as active galactic nuclei, whose engines can convert a galaxy’s gravitational and rotational energy into jets of particles that ultimately produce very-high-energy neutrinos. Observations in the ensuing years firmed up the case that the highest-energy neutrinos are born outside the Milky Way, but no specific extragalactic neutrino source had been identified.
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