An international team with Geneva participation reveals for the first time the origin of neutrinos, these elementary particles that reach our planet from the depths of the Universe. They would be born in particular in blazars, galactic nuclei fed by supermassive black holes.
Neutrinos, neutral particles that are difficult to detect, are highly energetic. They come from deep space and have traveled billions of light years before reaching our planet. Their mass is almost zero and they barely interact with matter, hence their nickname “ghost particle”. Neutrinos can pass through galaxies, planets and the human body almost without a trace, the University of Geneva has reported (UNIGE).
“Neutrinos are produced exclusively during processes involving the acceleration of cosmic rays,” says Sara Buson, professor of astrophysics at the Julius-Maximilians-Universität (JMU) in Würzburg, Germany, who co-piloted this study with the UNIGE. A research which was published in the journal Astrophysical Journal Letters.
Despite the vast amount of data collected on the subject, the link between high-energy neutrinos and the astrophysical sources that produce them remains largely a mystery. It was in 2017 that Sara Buson and her team for the first time integrated the idea that a blazar – the one named TXS 0506+056, more than 4 billion light-years away – might be a supposed source of neutrinos. These particles are thus unique messengers: thanks to them, it will be possible to locate the source of cosmic rays.
>> Neutrinos can help spot where blazars are in the Universe, like in this map:
A card from the catalog of multi-frequency blazars. [The Roma-BZCAT – www.ssdc.asi.it/bzcat5]
Near the speed of light
Blazars are active galactic nuclei powered by supermassive black holes that emit far more radiation than their entire galaxy (read box). Andrea Tramacere, researcher in the Department of Astronomy at UNIGE, carried out numerical modeling of acceleration processes and radiation mechanisms in this field. These flows of accelerated matter can approach the speed of light.
“The accretion process and the rotation of the black hole lead to the formation of relativistic jets, where the particles are accelerated and emit radiation of up to energies of a trillion times that of visible light”, notes the specialist .
The research team superimposed neutrino data obtained by the Neutrino Observatory IceCube in Antarctica – the most sensitive neutrino detector currently in operation – and the BZCATone of the catalogues of the most accurate blazars: “With these data, we had to prove that the blazars whose directional positions coincided with those of the neutrinos were not there by chance”, adds Andrea Tramacere.
Solid evidence
To do this, the UNIGE researcher has developed software capable of estimating how similar the distributions of these objects in the sky are: “After rolling the dice several times, we discovered that the random association does not can exceed that of the actual data only once in a million trials! This is strong evidence of the correctness of our associations,” he says.
Despite this unprecedented degree of certainty, the team believes that this first sample of objects is just the tip of the iceberg: “What we need to do now is understand what is the main difference between the objects which emit neutrinos and those which do not”, concludes the Geneva researcher.
“This will help us understand how well the environment and the accelerator ‘dialogue’. We can then rule out certain models, improve the predictive power of other models, and finally add new pieces to the eternal cosmic ray acceleration puzzle!”
An acceleration that is still not yet well understood by physics and astrophysics.
>> A video explaining how neutrinos are detected by IceCube:
Stéphanie Jaquet and the ats
