For the first time ever a possible source for cosmic neutrinos has been identified thanks to its association to a source of gamma rays with very high energies. The source is a so-called blazar, an active galaxy with an extremely massive black hole of several hundreds of milions of solar masses at its centre. It is located at a distance of 4.5 billion light years from the Earth in direction of the Orion constellation. The extraordinary discovery, published in Science, has become possible thanks to the combination of data with different origin: from the neutrino detector IceCube, taking data at the South Pole, and from another 15 experiments detecting photons on the Earth and in Space.
Important contributions to the discovery have been made by the researchers of the National Institute of Astrophysics (INAF), the National Institute for Nuclear Physics (INFN), the Italian Space Agency (ASI) and various Italian universities that are involved in these experiments.
The result has been made possible by the “choral” work at the frontier of astronomy, the multimessenger astronomy, which not only observes electromagnetic radiation at all wavelenghts (radio, microwave, infrared, visible, UV, X-rays and gamma) but also gravitational waves, cosmic rays and neutrinos. The discovery is a solid indicator towards the solution for one of the greatest enigmas of astrophysics: the origin of cosmic rays with extremely high energies.
The turning point towards this extraordinary discovery dates back to September 2017, when the IceCube detector observed for the first time an interesting neutrino which was then given the name IC-170922A. Its very high energy of 290 TeV (terraelectronvolt, one thousand billions of electronvolt) indicated that most probably its origin had to be a very “active” distant object. It was expected that the production of cosmic neutrinos was accompanied by gamma rays. For this reason, right after the identification of the neutrino, IceCube launched a “neutrino alert” to all telescopes operating on Earth and in Space. The hope was that their observations could help identify with precision the source. And indeed this was the case.
The Fermi satellite, realized by NASA with an important participation of the National Institute of Astrophysics (INAF), the National Institute of Nuclear Physics (INFN) and the Italian Space Agency (ASI), succeeded in localising the emission of gamma radiation coinciding with a known source with the precision of one tenth of a degree by observing with its Large Area Telescope (LAT) the extremely high energy gamma rays in the direction of the neutrino source. The source was a blazar with an Active Galactic Nucleus (AGN), that is an extremely massive black hole at the center of a galaxy, that expels a jet of radiation and matter in our direction at almost the velocity of light. Also Fermi-LAT launched an alert right away. This allowed for several experiments to point to the source. Amongst these were the MAGIC telescopes. Turning their gigantic mirrors towards the identified source they can measure the emission spectrum at an energy which is a thousand times higher than the spectrum observed by Fermi-LAT, adding this way another important piece to this discovery.
Thanks to all these different observations, it was possible to identify the blazar TXS 0506+056 as the source of neutrino IC-170922A. The distance of the galaxy from the Earth was measured by a group of researchers lead by the Padova divison of INAF.
The observation not only of the gamma rays but also of a high energy neutrino allows to assert that the AGN jets contain also extremely high energy protons. The IC-170922A neutrino helps to partially solve the great mystery about the very high energy cosmic rays and confirms the idea of a strong relation between the various cosmic messengers.
The study of cosmic rays has a long tradition in Padova, dating back to the 30s when Bruno Rossi started to investigate this field of research. Also the local division of the INFN has been actively involved from its first days on. The understanding of the origin of cosmic rays and that of the messengers of those regions in the Universe where, under extreme conditions, particles of extraordinary energies are produced and accelerated, has motivated the Padova INFN division to get involved since the beginning in the Fermi-LAT and MAGIC projects, with the latter studying the gamma rays originating from celestial sources from Earth through the detection of Cherenkov radiation.
This particular scientific discovery has been made possible also thanks to the work of researchers from Padova, both on the identification of the neutrino source as well as on the study of the spectrum of the emitted electromagnetic radiation at highest energies with the MAGIC telescopes.
Also worth mentioning is the remarkably faresighted work of Milla Baldo Ceolin in Padova. She gave significant contributions to this field of research, starting also an important series of workshops on these topics aimed at discussing its prospectives. The first “International Workshop on Neutrino Telescopes: another way to observe the universe” was held in 1988 at the Istituto Veneto di Scienze, Lettere ed Arti in Venice. Today the conference series has arrived at its 18th edition, carried on by her students and research collaborators.
- Artistic reproduction of the observed event.
- Artistic visualisation of the observation.
- The MAGIC telescopes. (Credits: IAC, http://www.iac.es/copyright.php?lang=en)
- The Italian researchers of the MAGIC collaboration.
- The FERMI experiment (Credits: NASA https://www.nasa.gov/multimedia/guidelines/index.html)
- One of the MAGIC telescopes.