Three years ago, in the waters of the Mediterranean Sea, the passage of an "ultra-energetic" cosmic neutrino was observed — the most energetic ever detected. The event drew international attention from the scientific community as well as from the media and the public, not least because the origin of this particle — whose energy exceeded that of previously observed neutrinos by more than an order of magnitude — is unknown.
A new paper published in the Journal of Cosmology and Astroparticle Physics (JCAP) by the KM3NeT collaboration, which operates the KM3NeT/ARCA detector off the coast of Sicily, suggests that the source of this particle may be in a population of blazars — active galactic nuclei hosting a supermassive black hole that emit a plasma jet directed toward Earth.
In search of the "culprit"
KM3NeT/ARCA is a neutrino detector immersed in the depths of the sea off the coast of Sicily, and it may come as a surprise that it is still under construction. Nevertheless, on 13 February 2023 it recorded an extraordinary signal: the passage of a neutrino which, at around 220 PeV, far exceeded the energies of all high-energy neutrinos observed up to that point. The event also caught the scientific community off guard: what could have generated a particle with such exceptional characteristics?
To answer this question, the collaboration worked much like forensic investigators deducing who or what left a particular trace at a crime scene: starting from an initial hypothesis, the authors simulated the events that might have occurred and then compared the results with the actual observations.
The authors' hypothesis — one among several proposed over the past year — is that the ultra-high-energy neutrino may have been produced in a specific class of blazars. "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."
A diffuse source
Bendahman explains that there are reasons to believe the observed neutrino did not originate from a single sudden and identified event — such as an explosion or a flare. In similar cases, scientists look for an electromagnetic "counterpart," that is, a signal in radio, optical, X-ray or gamma-ray emission coming from the same region of the sky in coincidence with the neutrino detection
In the case of the event three years ago, however, no such electromagnetic counterpart was found. "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."
"We therefore simulated a population of blazars using an open-source software called AM3, with physically motivated parameters," Bendahman explains. To build a realistic model of blazars, the researchers fixed many parameters to values already known from other independent observations, such as the magnetic field strength or the size of the emission region.
In the simulations, they mainly varied two key parameters: the baryonic loading, which indicates how much energy is carried by protons compared to electrons (and therefore how many neutrinos can be produced), and the proton spectral index, which determines how the proton energy is distributed and how likely it is to reach extreme energies.
For each combination of these two parameters, they calculated both the diffuse neutrino flux and the corresponding gamma-ray flux, to be compared with observational data.
The comparison with IceCube and Fermi LAT
One of the strengths of Bendahman and colleagues' work is its integrated approach: in addition to KM3NeT/ARCA data, the authors also considered observations from the IceCube Neutrino Observatory and the Fermi Gamma-ray Space Telescope. They did not rely only on what had been observed, but also — and perhaps especially — on what these instruments had not observed.
The absence of comparable ultra-high-energy events in existing neutrino datasets, including those from IceCube, suggests that such phenomena are extremely rare. Any viable model must therefore also account for this absence. The proposed scenario satisfies this constraint.
Moreover, since neutrino production is generally accompanied by gamma-ray emission, the authors verified that the contribution from blazars does not exceed the extragalactic gamma-ray background measured by Fermi.
In this way, Bendahman and colleagues showed that a population of blazars is a plausible source of the ultra-high-energy neutrino: "We modelled 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."
KM3NeT: the best is yet to come
The hypothesis that a population of blazars may lie at the origin of the event remains promising, but it needs to be tested with new data. "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." At the time of the observation, only 21 detection lines of KM3NeT were active, corresponding to about 10% of the final volume of the apparatus.
If confirmed, this KM3NeT collaboration's interpretation would provide new insights into the ability of blazars to accelerate particles to even more extreme energies than previously hypothesized. "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."
