Record-energy neutrino detected off Sicily may trace back to a population of blazars

The most energetic neutrino ever observed arrived on February 13, 2023, streaking through the deep Mediterranean waters off Sicily at roughly 220 peta-electronvolts (PeV). That is more than ten times the energy of any neutrino previously recorded. The KM3NeT/ARCA detector that caught it was, at the time, only about 10% complete. And nobody could say where the particle came from.

Now, a study published in the Journal of Cosmology and Astroparticle Physics offers a candidate answer: the neutrino may have originated not from a single cataclysmic event, but from the collective output of a population of blazars, active galactic nuclei that aim jets of plasma directly at Earth from the vicinity of supermassive black holes.

No flash, no counterpart

When astrophysicists detect a neutrino of unusual energy, they typically look for an electromagnetic counterpart: a burst of light in radio, optical, X-ray, or gamma-ray wavelengths coming from the same patch of sky at roughly the same time. Such a signal would point toward a specific source, perhaps a supernova, a flaring blazar, or a tidal disruption event.

In this case, no counterpart was found. That absence does not rule out a single source entirely, but it shifts attention toward a different explanation: the neutrino may belong to a diffuse background flux, the cumulative output of many sources rather than one dramatic event.

Building a blazar population from scratch

Meriem Bendahman, a researcher at INFN Naples and a member of the KM3NeT collaboration, explains the team's approach. Using an open-source simulation tool called AM3, the researchers constructed a model of a blazar population with parameters grounded in existing observational data. Magnetic field strengths, emission region sizes, and other physical properties were fixed to values already established by independent measurements.

The team then varied two key parameters: the baryonic loading, which governs how much energy is carried by protons relative to electrons (and thus how many neutrinos can be produced), and the proton spectral index, which determines how proton energy is distributed across different energy scales.

For each combination of these parameters, the researchers calculated both the expected neutrino flux and the associated gamma-ray emission. The resulting predictions needed to satisfy two constraints simultaneously.

Cross-checking with IceCube and Fermi

The analysis did not rely on KM3NeT data alone. The team also compared their model against observations from the IceCube Neutrino Observatory at the South Pole and the Fermi Gamma-ray Space Telescope. The logic worked in two directions.

First, the rarity of the detection matters. IceCube has been operating for over a decade with a much larger instrumented volume, yet it has never recorded a neutrino at comparable energy. Any viable source model must account for this absence. The blazar population scenario does: such extreme events would be rare enough to have plausibly escaped IceCube's reach.

Second, neutrino production in blazars is accompanied by gamma-ray emission. The blazar model's predicted gamma-ray output had to remain consistent with the extragalactic gamma-ray background measured by Fermi. It did.

The result: a blazar population with physically motivated parameters can explain the 220 PeV neutrino while remaining compatible with everything else we have observed, and everything we have not.

A detector still taking shape

Perhaps the most remarkable aspect of this story is the instrument at its center. When KM3NeT/ARCA recorded the ultra-high-energy event, only 21 of its detection lines were active, a fraction of the final planned configuration. The detector is being built on the seafloor in stages, with strings of optical modules lowered into position to watch for the faint Cherenkov light that neutrinos produce when they interact with water.

Once completed, the full detector will have a dramatically larger effective volume. That means more neutrino detections, better directional resolution, and the statistical power to distinguish between competing source models.

Other hypotheses on the table

Blazars are not the only explanation being considered. Some researchers have proposed that ultra-high-energy cosmic rays interacting with the cosmic microwave background, the remnant radiation from the early universe, could produce neutrinos at these energies through a process known as the GZK mechanism. Other models invoke different classes of cosmic accelerators.

The blazar hypothesis has the advantage of being testable with the data in hand and consistent with existing observations. But confirming it will require more events, and those events will require more detector.

If the blazar interpretation holds up, it would push the known boundaries of particle acceleration in these objects. Blazars were already understood to be powerful accelerators, but producing neutrinos at 220 PeV implies proton energies beyond what most models had previously considered realistic.

The short answer to where the most powerful neutrino ever seen came from: we don't know yet. But the list of plausible suspects just got shorter.