Neutrino factories in space

Extremely energetic and difficult to detect, 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) clarified one aspect of the mystery: neutrinos are born in blazars, fed by galactic nuclei supermassive black holes. These results are published in the journal Astrophysical Journal Letters.

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

The birthplace of cosmic rays produces neutrinos. Neutrinos are neutral particles that are difficult to detect. They have almost no mass and interact very little with matter. They move through the universe and can travel through galaxies, planets and human bodies almost without a trace. “Astrophysical neutrinos are produced exclusively in processes involving the acceleration of cosmic rays,” explains Sarah Busson, professor of astrophysics at the Julius-Maximilians-Universität (JMU) Würzburg in Bavaria, Germany. This is precisely what makes the neutrino a unique messenger to detect cosmic ray sources.

A step forward in controversial debates

Despite the vast amount of data that astrophysicists have collected, the relationship of high-energy neutrinos to the astrophysical sources that produce them has been an unsolved problem for years. Sarah Busson always finds it a big challenge. It was in 2017 that the researcher and his collaborators first presented a blazar (TXS 0506+056) in discussion as a putative neutrino source in the journal Science. Bluzars are active galactic nuclei powered by supermassive black holes that emit far more radiation than their entire galaxy. The publication sparked a scientific debate over whether there really is a link between blazars and high-energy neutrinos.

Following this encouraging first step, Professor Busson’s group launched an ambitious multi-messenger research project in June 2021 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 origin of astrophysical neutrinos, possibly establishing the blazar as the first source of high-energy extragalactic neutrinos.

The project is now experiencing its first success: in the journal Astrophysical Journal Letters, Sarah Busson, with her group, former postdoc Ranier de Menezes (JMU) and Andrea Tramassere of the University of Geneva, reports that blazars can be confidently associated with astrophysical. Neutrinos are confirmed to an unprecedented degree.

Reveal the role of blazers

Andrea Tramassere is an expert in numerical modeling of acceleration processes and radiation processes working on relativistic jets — flows of accelerated matter, near the speed of light — especially in blazar jets. “The process of growth and rotation of black holes leads to the formation of relativistic jets, where particles are accelerated and emit radiation up to one trillionth of the power of visible light! The discovery of the link between these objects and cosmic rays could be the “Rosetta stone” of high-energy astrophysics! »

To reach these results, the research team used neutrino data from the IceCube neutrino observatory in Antarctica – the most sensitive neutrino detector currently in operation – and BZCat, one of the most sensitive catalogs of blazars. correct “With these data, we had to prove that the directional positions of the blazars corresponded to those of the neutrinos. To do this, UNIGE researchers developed a software capable of estimating how similar the distribution of these objects is in the sky. “After rolling the dice several times, we discovered that random association can outperform the original data only once in a million trials! This is solid proof that our association is correct. »

Despite this success, the research team believes that this first sample of the object is only the “tip of the iceberg”. This work allowed them to collect “new observational evidence”, which is the most important element for creating a more realistic model of the astrophysical accelerator. “What we need to do now is understand what is the main difference between objects that emit neutrinos and those that don’t. This will help us understand how much the environment and accelerator “talk” to each other. We will then be able to rule out some models, improve the predictive power of others, and finally, add more pieces to the eternal puzzle of cosmic ray acceleration! »

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Materials provided by University of Geneva. Note: Content may be edited for style and length.

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