Origin of a rare cosmic neutrino traced

A major result in multi-messenger astronomy — where several separate observations are combined to provide insight into astrophysical events — has been announced today in an international press conference: first evidence for an extragalactic object acting as a source of high-energy neutrinos. These results are also reported in two papers that have been published today in the journal Science.

Neutrino triggers multi-messenger observations

Neutrinos are elementary particles that barely interact with matter. Nonetheless, neutrinos are important cosmic messengers that carry unique information about the regions of the Universe in which they were produced. The largest detector specialised in hunting neutrinos is IceCube, located at the South Pole. It detects some 200 neutrinos every day. Most of these, however, have low energies and are produced by cosmic rays interacting with the Earth’s atmosphere. Therefore the excitement when, on 22 September 2017, IceCube detected a very-high-energy neutrino (of roughly 290 TeV energy), indicating that the particle originated from a distant celestial object.

To investigate the origin of that special neutrino, and to learn more about its source, IceCube issued an alert to several instruments that can detect with high sensitivity another signature that, according to theoretical models, should accompany the emission of neutrinos: high-energy gamma rays. And indeed, the satellite-based Fermi gamma-ray space telescope found increased gamma-ray activity that may have had the same origin as the neutrino. This additional information served then as necessary input to point the Earth-based Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes at the Roque de los Muchachos Observatory on the Canary island of La Palma in the right direction, to gather further information about the possible source of the neutrino.

Taking all the data obtained by IceCube, the Fermi space telescope and the ground-based MAGIC telescopes together — and considering in particular the time correlation between the different observations — a so-called blazar known as TXS 0506+056 emerged as a likely candidate for the neutrino source.

Hints towards the origin of cosmic rays

This connection to TXS 0506+056 is important. As the birth of neutrinos is always linked to proton interactions, these multi-messenger observations may help to solve the old mystery regarding the origin of cosmic rays, which has been first discovered in 1912 and largely consist of high-energy protons. These protons, however, are deflected by magnetic fields in space, making it nearly impossible to pinpoint their place of origin once they reach Earth. In contrast, neutrinos and gamma-ray photons travel through the universe without much deflection, which means that their origin can much more reliably determined. Now having observed a correlation between gamma-ray observations and a neutrino-detection event — related to the production of high-energy protons — provides strong additional evidence that blazars like TXS 0506+056 contribute to the flux of extragalactic cosmic rays.

An international effort

To obtain these breakthrough results has only been possible thanks to three major international collaborations: Ice Cube, the Fermi satellite and the MAGIC telescopes.

One of the MAGIC collaboration members is the group of Professor Adrian Biland at the Institute for Particle Physics and Astrophysics at ETH Zurich, and previously also the group of Professor Felicitas Pauss, who retired in 2016. The ETH group concentrates on different scientific questions investigated with MAGIC and was not directly involved in the studies reported today, but with their responsibility for the Active Mirror Control unit — which maintains the optical quality of the telescope while it deforms under its own weight when positioned — they have an important role in ensuring that MAGIC delivers the sort of high-quality data needed on pioneering studies such as the one presented today.

(Text based on a external page MAGIC media release.)