Research by the KM3NeT collaboration suggests that the extreme neutrino detected in the Mediterranean Sea in 2023 did not come from a single event, but rather from a diffuse flux of particles generated in a population of active galactic nuclei known as blazars.
An unprecedented discovery of neutrinos in the Mediterranean Sea has expanded the boundaries of high-energy astrophysics, raising new questions about the most extreme process in the universe.
Three years ago, scientists discovered an “ultra-energetic” cosmic neutrino in the Mediterranean Sea, the most energetic ever recorded. The discovery attracted worldwide attention from researchers, the media and the public. One reason for the great interest is that the particle’s origin remains unknown. Its energy was more than ten times greater than any neutrino previously observed.
Research by the KM3NeT collaboration suggests a possible explanation: the particle may have come from a population of blazars, active galactic nuclei powered by supermassive black holes that shoot jets of plasma toward Earth.
Looking for the "culprit"
The team operates KM3NeT/ARCA, a deep-sea neutrino detector off Sicily that is still under construction. On 13.2.23, it picked up an unusual signal. The energy of the detected neutrino was about 220 PeV (about 35 joules), much higher than any other high-energy neutrino previously measured. The finding surprised scientists and raised an important question: What kind of source could produce such an extreme particle?
To investigate, the researchers took an approach similar to forensic analysis. They started with possible explanations, ran simulations of those scenarios, and compared the results with the real data.
One of the leading ideas was that the neutrinos came from a certain type of blazar. "There are several possible explanations for the origin of this particle," explained Mariam Bandaman. "For example, it has been proposed that such neutrinos are created by the interaction of ultra-energetic cosmic rays with the cosmic microwave background radiation, the residual light from the early universe. But there is also the possibility that the neutrinos originate in a diffuse flux generated by a population of extreme accelerators, such as blazars."
Diffuse source rather than a single event
Bandaman and her colleagues found clues that the neutrinos did not come from a single dramatic event like an explosion or outburst. In such cases, scientists typically look for an electromagnetic “correspondent,” that is, a signal in radio, optical, X-ray or gamma-ray wavelengths from the same area of the sky at the same time.
No signal was detected in this event. "This doesn't completely rule out the possibility of a point source," says Bandaman, "but it leads us to think that our neutrinos came from a diffuse source, that is, a flux of neutrinos that includes contributions from many sources."
To test the idea, they computer simulated a population of blazars and based many outputs on existing observations, such as the strength of magnetic fields and the size of the emission region.
They focused on two main variables: the baryonic charge, which describes how much energy protons carry compared to electrons, and the proton spectral index, which determines how the proton energy is dispersed. These factors affect the number of neutrinos produced and their energy.
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