IceCube pinpoints an extragalactic neutrino source

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.

Now one has: a galaxy named TXS 0506+056 about 4 billion light-years distant. A member of a group of active, variable galaxies known as blazars, TXS 0506+056 hosts a supermassive black hole and shoots a relativistic jet of plasma more or less in the direction of Earth.

The figure shows the event that pointed to TXS 0506+056, observed by IceCube on 22 September 2017. The colored circles indicate the firings of Cherenkov detectors; purple detectors fired first, yellow detectors three microseconds later. The straight line shows the reconstructed muon path. The IceCube collaboration estimates that the neutrino triggering the event had an energy of about 300 TeV.

In April 2016 IceCube established a system to rapidly notify astrophysicists of candidate astrophysical neutrino events. Within a minute of the 2017 measurement, the system broadcast to obervers an alert that included an estimate of the direction to the neutrino source. A refined estimate was sent out about four hours later. A score of telescopes—radio through gamma ray—responded to the alert; their observations established that the IceCube-supplied coordinates corresponded to TXS 0506+056. The Large Area Telescope aboard the Fermi Gamma-Ray Space Telescope, which had been surveying the entire sky since August 2008, reported a spike in the galaxy’s gamma activity at the time of the IceCube observation. And all across the electromagnetic spectrum, telescopes saw significant variability in their signals, an indication that TXS 0506+056 was active.

The observation of the 2017 event prompted the IceCube team to revisit the data they had taken since April 2008 in search of evidence of other neutrinos coming from the direction of TXS 0506+056. They found that in a five-month period in 2014–15 the IceCube detector had seen a neutrino flux 3.5 σ in excess of atmospheric background. The researchers emphasize that the 2014–15 detection is independent of the evidence presented in connection with the 2017 event and therefore confirms their hypothesis that TXS 0506+056 is a source of very-high-energy neutrinos. (IceCube, Fermi-LAT, MAGIC, and other teams, Science 361, eaat1378, 2018 ; IceCube collaboration, Science 361, 147, 2018 .)