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Sunset at the North Campus © Roland Baege​/​TU Dortmund
HOT LEAD IN THE PERPETUAL ICE

UA Ruhr Plays Leading Role in Pioneering Observations of Neutrino Sources

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  • Particle Physics
  • Research
South Pole building © Archiv​/​TU Dortmund
The scientists in the IceCube collaboration are studying neutrinos from the cosmos with the help of detectors sunk kilometers deep into the ice. (Archive/TU Dortmund University)
Using detectors sunk kilometers deep into the ice, scientists at the South Pole are searching for neutrinos from the cosmos. They hope to gain new insights into the Universe by observing these highly penetrating elementary particles. In 2013, IceCube, the most powerful detector for high-energy neutrinos worldwide, achieved an initial breakthrough: The researchers were able to detect a flux of neutrinos outside the solar system. At first, however, the sources of these neutrinos were unknown. This could now change.

Together with the other IceCube partners, researchers from the University Alliance Ruhr (UA Ruhr) have made spectacular observations that could help to clearly identify neutrino sources. In particular, the scientists from TU Dortmund University, Ruhr-Universität Bochum (RUB) and the University of Duisburg-Essen contributed to the complex analysis and interpretation of the data from the perpetual ice as well as to simulation calculations. The corresponding articles will appear this week in the prestigious journal Science.

Traces of light in the ice might originate from the galactic nucleus “TXS 0506+056”

In their rare interactions, the high-energy neutrinos from the cosmos sometimes generate charged particles, which in turn leave a telltale trail of light in the ice. If IceCube’s sensors detect such a trail, it is then possible to determine the neutrino’s direction of origin. This is precisely what happened on 22 September 2017. IceCube registered event 170922A, and various telescopes around the world rapidly honed in on the neutrino’s trail.

To study such high-energy processes in the Universe, researchers from UA Ruhr have joined forces in the “Ruhr Astroparticle-Plasma Physics Center” (RAPP Center). “What is known as multi-messenger astronomy, that is, the comprehensive observation in various wavelength ranges and with all available media, has meanwhile become a very important source of knowledge and a particular strength of the working groups in the RAPP Center,” explains Professor Julia Tjus from RUB, one of the authors of the studies now presented. Indeed, it quickly transpired that the astrophysicists’ efforts might be successful. At the celestial position at which the neutrino was detected, the research teams discovered a source of gamma radiation: the active galactic nucleus “TXS 0506+056”, which surrounds a massive black hole. This object was already known from previous observations. Since, however, there is a relatively large number of known sources of gamma rays in the Universe, the researchers had to find out whether TXS 0506+056 could be the source of the high-energy neutrinos.

TU Dortmund University is observing gamma rays within an international collaboration

“This is the point where our MAGIC telescopes on La Palma come into play which, just like the IceCube detector, we have been operating within an international collaboration for several years. Several university partners are leading the collaborative research, which is funded by Germany’s Federal Ministry of Education and Research,” explains Professor Wolfgang Rhode from TU Dortmund University. These telescopes facilitate observations of cosmic gamma-ray sources at high energies. It was already clear after only a few days that the gamma-ray source TXS 0506+056 could also be detected for the first time in the energy range of the MAGIC telescopes. “Detecting a gamma-ray source in the neutrino’s direction of origin, which had not been observed before in this energy range, is a very important finding. This is precisely the first concrete sign of the possible discovery of a neutrino source that we had expected,” adds Professor Rhode.

The scientists might possibly have achieved a breakthrough in high-energy astrophysics: “Identifying an active galactic nucleus as a neutrino source would not only be a major step toward understanding the processes in the immediate surroundings of massive black holes but also a hot lead to the origin of mysterious high-energy cosmic radiation,” explains Julia Tjus.

RAPP groups are well-equipped for further research

The task now is to confirm the suspicion that the neutrino originates from an active galactic nucleus by evidencing further events. The RAPP groups are in an excellent position to perform these exciting tasks: “Through the stable and continued operation of the MAGIC telescopes as a supplement to IceCube and by participating in the future gamma-ray observatory ‘Cherenkov Telescope Array’ as well as the IceCube expansion ‘Gen-2’, the RAPP groups can continue to play a key role,” explains Dr. Dominik Elsässer from TU Dortmund University. His colleague Dr. Tim Ruhe adds: “In modern astrophysics and particle physics, intelligent analysis methods and the simulation of detectors on mainframes play an essential role. It is in this area that we are making leading-edge contributions to both IceCube and MAGIC.” Much of the work on innovative and intelligent analysis methods is being conducted within Collaborative Research Center 876 “Availability of Information through Analysis under Resource Constraints” at TU Dortmund University.

Another piece of the puzzle that is helping to identify neutrino sources is the interpretation of the data generated by the different measurements. “For this, we are developing numerical models here in Bochum that describe the physics of neutrino sources as accurately as possible. With the help of these models, we can predict how the various wavelengths and different types of particle achieve a certain intensity and chronological sequence,” explains Dr. Björn Eichmann from RUB. This precise modeling is a specialty of the RAPP Center, as it can make use of the expertise of the local astrophysics, plasma physics and particle physics experts – a locational advantage that is unique in Germany.

The UA Ruhr groups are also introducing their strengths directly into the observations: Alicia Fattorini, a doctoral candidate at TU Dortmund University, is part of a team in the MAGIC collaboration that performs rapid analyses of neutrino follow-

up observations. Johannes Werthebach, who is also a doctoral candidate in Dortmund, is currently at the South Pole and supervising the IceCube detectors.

University Alliance Ruhr

Ruhr-Universität Bochum (RUB), TU Dortmund University and the University of Duisburg-Essen have been working closely and strategically together under the umbrella of UA Ruhr since 2007. By joining forces, the partner universities’ performance is systematically upgraded. Reflecting the motto “better together”, there are meanwhile over 100 cooperative projects in research, teaching and administration. With over 120,000 students and almost 1,300 professors, UA Ruhr is one of Germany’s largest and top-performing centers of science and research.

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