A JCAP paper models one possible origin for the 220 PeV particle, a population of blazars, but the case is open, and the unfinished KM3NeT/ARCA telescope is the most important variable in what comes next.
On the morning of 13 February 2023, a single particle crossed the seafloor off the coast of Sicily carrying roughly 220 petaelectronvolts of energy, more than an order of magnitude beyond any neutrino previously recorded. The detector that caught it, KM3NeT/ARCA, was not finished. Only 21 of its planned detection lines were live, the collaboration said, meaning the apparatus was running at about a tenth of its eventual volume. A new analysis published this month asks where that particle came from, and treats the question the way a forensic team would: by building a lineup of suspects, simulating what each would have produced, and checking the predictions against everything else the sky has shown us.
The event, catalogued as KM3-230213A, was first reported in detail in Nature on 12 February 2025. The new work is a separate paper, "Blazars as a Potential Origin of the KM3-230213A Event," published in the Journal of Cosmology and Astroparticle Physics on 9 March 2026. The lead author is Meriem Bendahman, a researcher at INFN Naples and a member of the KM3NeT collaboration, a project of more than 360 scientists from 22 countries. The lead suspect in the new paper is a class, not a single object: blazars, a subset of active galactic nuclei in which a supermassive black hole drives a plasma jet almost directly at Earth.
The team did not pick blazars by intuition. They used AM3, an open-source simulation tool, and varied two physically meaningful parameters across ranges fixed by independent observations: the magnetic field strength at the emission site, and the proton spectral index, the slope of the energy distribution of the protons being accelerated in the jet. For each parameter choice, AM3 produced a predicted neutrino flux. The researchers then asked two sharp questions of the result. Would a blazar population that emits this neutrino flux also have produced other ultra-high-energy neutrinos that IceCube, the larger and longer-running neutrino observatory at the South Pole, should have seen? And would the same population produce a gamma-ray background consistent with what NASA's Fermi Gamma-ray Space Telescope actually measures? The blazar scenario, according to the authors, satisfies both constraints simultaneously.
That cross-checking matters because a single smoking gun was missing. No radio, optical, X-ray, or gamma-ray counterpart lit up at the neutrino's arrival time and direction. The lack of a coincident electromagnetic flash is one reason the team modeled a diffuse population of blazars rather than pointing at a single identified transient. If the neutrino had come from a specific flare of a specific galaxy, some telescope should have seen something.
A second candidate remains on the table. Cosmogenic neutrinos are produced when ultra-high-energy cosmic rays collide with the cosmic microwave background during intergalactic travel. That mechanism is also a viable origin for a 220 PeV particle, and the JCAP paper does not claim to rule it out. Bendahman's quote to EurekAlert is careful on this point: the blazar population, she said, "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." Could, not does. The paper is a structured set of testable hypotheses, not a closed case.
The scale of the finding is easier to grasp with an anchor. The Max Planck Society notes that the neutrino carries about 16,000 times the energy of the most powerful particle collisions produced at CERN's Large Hadron Collider. "Most energetic" here is a claim about a specific instrument, KM3NeT/ARCA, and the collaboration that runs it, not a universal cosmic record independent of which detector happened to be listening.
What changes when the detector finishes is the most concrete thing the reader can take away. KM3NeT/ARCA is still under construction, and at the time of the 2023 event it was operating with a small fraction of its planned detection lines. The team needs more events, across a range of energies and arrival directions, to distinguish between candidate source populations with any statistical confidence. "We need more observational data," Bendahman said. "KM3NeT is still under construction, and we detected this ultra-high-energy neutrino with only a partial configuration." A finished detector does not just catch more of the same. It catches the second event, and the third, and the rarer lower-energy neighbors that turn one spectacular outlier into a population.
A neutrino at 220 PeV arrived, the collaboration's own framing suggests it is the most energetic the detector has ever seen, and KM3NeT/ARCA was barely listening at the time. The case is open. The instruments are still being built.