An international team of scientists hunting for the origin of a mysterious ultra-energetic cosmic neutrino detected in 2023 said new evidence suggests the unusually powerful particle, whose energy exceeded that of previously observed neutrinos by more than an order of magnitude, may be the product of a population of blazars.
According to the KM3NeT collaboration research team that operates the KM3NeT/ARCA detector off the coast of Sicily, where the particle was observed, the suspect blazars are active galactic nuclei hosting supermassive black holes that emit plasma jets directed toward Earth.
If the new origin theory is confirmed, it would force astrophysicists to reconsider blazars’ ability to accelerate particles to even more extreme energies than those proposed in earlier theories.
Origin of Ultra-Energetic Cosmic Neutrino Remains a Mystery
Designed as the world’s largest and most sensitive neutrino detection facility, KM3NeT/ARCA is intentionally immersed in the deep sea off the coast of Sicily. Although it is still currently under construction, the facility made science history over three years ago.
According to a statement announcing the new blazar origin theory, the facility was only about 10% complete by volume on February 13th, 2023, when it detected the passage of a single neutrino. Although the facility was designed for such chance encounters, this detection registered an energy of 220 PeV. In the small set of previously detected high-energy neutrinos, the team said this “ultra-energetic” cosmic neutrino was the highest-energy particle of its kind ever detected.
Adding to the historic nature of the detection, which included significant media attention, was the mystery surrounding the particle’s origin. More specifically, the researchers who made the detection could not immediately determine where it had come from. This conundrum was echoed by the wider scientific community, which also failed to immediately identify the origin of a particle with what the KM3NeT/ARCA team described as “unusual characteristics.”
In the three years since, several potential origins have been proposed, including many in the last year alone. However, to date, none of the ideas has gained unanimous support. Now, after approaching the event like “forensic investigators deducing who or what left a particular trace at a crime scene,” the international team believes they have finally cracked the proverbial case.
‘Mysterious’ High Energy Neutrino Likely Originated from a Population of Blazars
According to study co-author Meriem Bendahman, a researcher at INFN Naples and a member of the KM3NeT collaboration, several possible explanations have been proposed to explain where the particle came from. This includes single-event hypotheses, such as an interaction between high-energy cosmic rays and the cosmic microwave background radiation left over from the Big Bang.
However, Bendahman notes, it is also possible that the highly energetic particle detected in February of 2023 may originate from a “diffuse flux produced by a population of extreme accelerators, such as blazars.” In the new paper, which credits hundreds of scientific collaborators, the team highlights some of the problems with single-event theories.
For example, when scientists detect cosmic explosions like those proposed, they will search for evidence of the same event across other electromagnetic spectra. This includes “counterpart” signals in either the radio, x-ray, or gamma spectrum. To date, no counterpart signals for the 2023 event have been located.
Bendahman noted that this lack of corresponding signal from the area of the sky where the high energy neutrino originated was “does not completely rule out the possibility of a point-like source. However, the LNFN researcher, “it leads us to consider that our neutrino may come from a diffuse background,” such as a flux of neutrinos containing contributions from many other sources.
Computer Simulations and Previous Observations Hint at a Cosmic Conclusion
To test their hypothesis, the researchers used advanced simulation software called AM3, designed to mimic the conditions of cosmic phenomena like blazars. According to Bendahman, their experiments involved a population of simulated blazars “with physically motivated parameters,” derived from already known, independent observations. These included constraints for magnetic field strength and emission region size.
During the team’s simulations, they periodically varied the environment’s baryonic loading and the proton special index. According to their statement, the first parameter “indicates how much energy is carried by protons compared to electrons (and therefore how many neutrinos can be produced).” The second parameter determines “how the proton energy is distributed and how likely it is to reach extreme energies.”
To improve the overall simulation quality, the researchers included observations from the IceCube Neutrino Observatory and the Fermi Gamma-ray Space Telescope. Critically, the researchers note, they not only factored in these previous observations, but also focused on “what these instruments had not observed.”
As expected, this analysis found no corresponding events. The researchers said that this lack of comparable detection suggests that events like the February 2023 ultra-energetic cosmic neutrino detection are extremely rare, and that any viable model “must therefore account for this absence.”
“The proposed scenario satisfies this constraint,” they said.
“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,” the researchers write.
Unraveling How Objects Emit Particles at Energies Beyond What Was Expected
When discussing the implications of their blazar origin theory for the mysterious ultra-energetic cosmic neutrino detected in 2023, the researchers noted that “more observational data” will be needed before they can close the case. According to Bendahman, that final verdict will likely have to wait for the facility’s completion
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 detected this ultra-high-energy neutrino with only a partial configuration,” the researcher explained. “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.”
“We have never observed such a high-energy neutrino before,” the researcher added, “If it turns out to come from cosmic accelerators like blazars, it would give us new insight into how these objects can emit particles at energies beyond what we previously expected.”
The study “Blazars as a Potential Origin of the KM3-230213A Event” was published in the Journal of Cosmology and Astroparticle Physics.
Christopher Plain is a Science Fiction and Fantasy novelist and Head Science Writer at The Debrief. Follow and connect with him on X, learn about his books at plainfiction.com, or email him directly at christopher@thedebrief.org.