Neutrino factories in the deep outer space

A blazar accelerates cosmic rays

image: Beginning of a journey across the universe: The discovery of extragalactic neutrino factories
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Credit: © Benjamin Amend

Very energetic and hard to spot, neutrinos travel billions of light years before reaching our planet. Although it is known that these elementary particles come from the depths of our universe, their exact origin is still unknown. An international research team, led by the University of Würzburg and the University of Geneva (UNIGE), sheds light on one aspect of this mystery: neutrinos are thought to be born in blazars, galactic nuclei fed by supermassive black holes. These results are published in the journal Astrophysical journal letters.

The Earth’s atmosphere is constantly being bombarded by cosmic rays. These consist of electrically charged particles with energies up to 1020 electron volts. That is a million times more than the energy obtained in the world’s most powerful particle accelerator, the Large Hadron Collider near Geneva. The extremely energy-rich particles come from the deep outer space, they have traveled billions of light-years. Where do they come from, what shoots them through the universe with such tremendous force? These questions have been among the greatest challenges of astrophysics for over a century.

The birthplaces of cosmic rays produce neutrinos. Neutrinos are neutral particles that are difficult to detect. They have almost no mass and hardly interact with matter. They race through the universe and can travel through galaxies, planets and the human body almost without a trace. “Astrophysical neutrinos are produced exclusively in processes involving cosmic beam acceleration,” explains astrophysics professor Sara Buson of the Julius-Maximilians-Universität (JMU) Würzburg in Bavaria, Germany. This is exactly what makes these neutrinos unique messengers, paving the way for locating cosmic radiation sources.

A step forward in a controversial debate

Despite the large amount of data that astrophysicists have collected, the connection between high-energy neutrinos and the astrophysical sources derived from them has been an unsolved problem for years. Sara Buson has always considered it a great challenge. It was in 2017 that the researcher and collaborators first brought a blazer (TXS 0506 + 056) into the discussion as a presumed neutrino source in the journal Science. Blazars are active galactic nuclei driven by supermassive black holes that emit much more radiation than their entire galaxy. The publication sparked a scientific debate over whether there really is a connection between blazars and high-energy neutrinos.

After this first encouraging step, Prof. Buson’s group in June 2021 an ambitious multi-messenger research project with the support of the European Research Council. This involves analyzing various signals (“messengers”, such as neutrinos) from the universe. The main goal is to shed light on the origins of astrophysical neutrinos, possibly establishing blazars as the first source of high-security extragalactic high-energy neutrinos.

The project now shows its first success: In the Astrophysical Journal Letters magazine, Sara Buson reports with her group, former postdoc Raniere de Menezes (JMU) and with Andrea Tramacere from the University of Geneva, that blazars can certainly be linked to astrophysical neutrinos in a hitherto unprecedented degree of security.

Reveals the role of the blazers

Andrea Tramacere is one of the experts in numerical modeling of acceleration processes and radiation mechanisms that work in relativistic rays – outflows of accelerated matter approaching the speed of light – especially blazarjet aircraft. “The process of accretion and the rotation of the black hole lead to the formation of relativistic rays, where particles are accelerated and emit radiation up to a thousand billion energies of visible light! The discovery of the connection between these objects and the cosmic rays can be high energy astrophysics’ Rosetta. stone’!

To achieve these results, the research team used neutrino data from the IceCube Neutrino Observatory in Antarctica – the most sensitive neutrino detector in operation at the moment – and BZCat, one of the most accurate catalogs of blazers. “With these data, we had to prove that the blazars, whose directional positions coincided with the position of the neutrinos, were not there by chance.” To do this, the UNIGE scientist developed a software that was able to estimate how much the distribution of these objects in the sky is similar. “After rolling the dice several times, we discovered that the random association can only exceed the correct data once out of a million attempts! This is strong evidence that our associations are right.”

Despite this success, the research team believes that this first sample of objects is only ‘the tip of the iceberg’. This work has enabled them to collect “new observational evidence”, which is the main ingredient in building more realistic models of astrophysical accelerators. “What we need to do now is understand what the main difference is between objects that emit neutrinos and those that do not. This will help us understand the extent to which the environment and the accelerator ‘talk’ to then we will be able to exclude some models, improve the predictability of others, and finally add more pieces to the eternal puzzle of cosmic ray acceleration! “

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