Paper of the month:  Multi-messenger observations of a flaring blazar coincident with high-energy neutrino IceCube-170922A

December 13, 2018 by Bruno Martin

We live in an exciting era for astrophysics. Previously, our knowledge about astrophysical sources was mainly gained through electromagnetic radiation observations. Nowadays, we can also exploit observations of neutrinos and gravitational waves, and maximise our understanding by combining the signals from multiple sources. This is multi-messenger astronomy, a young and rapidly developing field, which promises to give insight on the universe at hugely different energy scales.

An important milestone has been reached with the identification of a high-energy neutrino’s astrophysical source.

The IceCube neutrino observatory searches for cosmic neutrinos using over 5000 optical modules (DOMs) embedded in the Antarctic ice. On 22 September 2017 the IceCube collaboration detected a high-energy neutrino, IceCube-170922A, with energy of hundreds of TeV. The neutrino’s energy and direction of origin were determined from the pattern of light seen by the DOMs (Figure 1).

As is standard for candidate astrophysical neutrinos with determined direction, IceCube alerted the astronomical community. Various groups began observations of the indicated patch of sky, and it was found to be consistent with the location of the known γ-ray blazar TXS 0506+056, and that the blazar was in a flaring state (Figure 2).

While other lower-energy neutrino origins have already been determined (the Sun and supernova SN1987A), localising the origin of high-energy neutrinos can shed light on questions such as the origin of other high-energy particles produced in the universe.

Neutrinos are produced alongside other cosmic ray particles through pion and muon decay. In turn, pions are produced by the scattering of hadrons. Conversely, photons are produced also via leptonic processes such as inverse Compton scattering. Therefore measuring the cosmic neutrino flux can discriminate between leptonic and hadronic sources, and hint to high-energy cosmic ray sources.

Figure 1: A display of the IceCube-170922A event: The size of the module indicates the light observed and the colour indicates the time since the first module hit. An arrow indicates the best fitted track direction. The inset shows the top view of the track.

Figure 2: Fermi-LAT and MAGIC observations at the location of the IceCube-170922A event. The track reconstruction was refined after the initial alarm, resulting in a small move of the neutrino location (green to peach square). The known gamma-ray source TXS 0506+056 is visible in both images.