Three years ago, scientists detected something extraordinary deep beneath the Mediterranean Sea: the most energetic cosmic neutrino ever observed. The particle carried an astonishing energy of around 220 PeV, more than ten times greater than previously detected high energy neutrinos, and researchers still do not know exactly where it came from.
Now, a new study published in the Journal of Cosmology and Astroparticle Physics (JCAP) suggests the particle may have originated from blazars, some of the universe’s most extreme objects. Blazars are active galactic nuclei powered by supermassive black holes that shoot enormous jets of plasma directly toward Earth.
Scientists Search for the Source of a Record Neutrino
The neutrino was detected on February 13, 2023, by KM3NeT/ARCA, a massive neutrino observatory located deep off the coast of Sicily. Interestingly, the detector is still being built. At the time of the discovery, only 21 detection lines were operational, representing about 10% of the observatory’s planned final size.
Even with its partial configuration, the detector captured a signal unlike anything scientists had seen before.
Researchers approached the mystery much like forensic investigators examining clues from a crime scene. Starting with one possible explanation, they created simulations and compared the results with the actual observations.
One leading idea is that the neutrino came from a special class of blazars capable of accelerating particles to extreme energies.
“There are several possible explanations for the origin of this particle,” explains Meriem Bendahman, a researcher at INFN Naples and a member of the KM3NeT collaboration, among the authors of the study, which counts hundreds of contributors. “For example, it has been proposed that such neutrinos are generated when ultra-high-energy cosmic rays interact with the cosmic microwave background radiation, the residual light from the early Universe. But there is also the possibility that the neutrino originates from a diffuse flux produced by a population of extreme accelerators, such as blazars.”
Why Blazars Became the Leading Suspects
In many cosmic events, astronomers search for an electromagnetic counterpart, such as radio waves, visible light, X-rays, or gamma rays coming from the same region of the sky at the same time as the neutrino detection.
But in this case, scientists found no matching signal.
“This does not completely rule out the possibility of a point-like source,” Bendahman notes, “but it leads us to consider that our neutrino may come from a diffuse background — that is, from a flux of neutrinos including contributions from many sources.”
That possibility pushed researchers toward the idea that the particle may have emerged from a large population of blazars rather than from a single dramatic cosmic event.
To investigate, the team used an open source simulation tool called AM3 to model realistic blazar populations. Many aspects of the simulations were based on values already measured through other observations, including magnetic field strength and the size of emission regions around the black holes.
The researchers mainly adjusted two critical factors. One was baryonic loading, which measures how much energy protons carry compared to electrons and helps determine how many neutrinos may be produced. The second was the proton spectral index, which affects how proton energies are distributed and whether they can reach extremely high energies.
For every simulation, the researchers calculated both neutrino production and the related gamma ray emission, then compared the results with real observations.
Comparing the Findings With IceCube and Fermi
The study combined observations from multiple major observatories, including KM3NeT/ARCA, the IceCube Neutrino Observatory, and NASA’s Fermi Gamma-ray Space Telescope.
Researchers did not just focus on what these instruments observed. They also considered what had not been observed.
For example, no other neutrino observatories, including IceCube, have detected similar ultra-high-energy events. That suggests particles like this are exceptionally rare, meaning any proposed explanation must also account for the lack of comparable detections.
The blazar model successfully matched that constraint.
The team also tested whether the proposed blazar population would produce too many gamma rays compared with the known extragalactic gamma ray background measured by Fermi. Their results remained consistent with existing observations.
In the end, the researchers found that a realistic population of blazars could plausibly explain the extraordinary neutrino event.
“We modeled a realistic population of blazars with physically motivated parameters, and we found that this population of blazars could explain the origin of this ultra-high-energy event, while also being consistent with the constraints that we have regarding the gamma-ray and neutrino observations,” Bendahman says.
KM3NeT Could Reveal Even More Extreme Cosmic Events
Scientists caution that more evidence is still needed before the blazar explanation can be confirmed.
“We need more observational data,” explains Bendahman. “KM3NeT is still under construction, and we detected this ultra-high-energy neutrino with only a partial configuration. With the full detector and more data, we will be able to perform more powerful statistical analyses and open a new window on the ultra-high-energy neutrino universe.”
If future observations support the theory, the findings could reshape scientists’ understanding of how blazars work and how powerful they can become.
“We have never observed such a high-energy neutrino before, and if it turns out to come from cosmic accelerators like blazars,” Bendahman concludes, “it would give us new insight into how these objects can emit particles at energies beyond what we previously expected.”