Three years ago, a single “ghost particle” soared into the Mediterranean with more energy than any ever observed before.
This ultra-energetic cosmic neutrino challenged previously held conceptions regarding the upper limits of neutrino energy. Its mysterious origins also drew global attention and speculation.
Now, a team of researchers in Italy has presented a new hypothesis, claiming the particle may have originated in a specific class of blazars. In a paper published in the Journal of Cosmology and Astroparticle Physics, they explain how they tested several hypotheses in simulations.
Ultimately, they found that the record-breaking neutrino may have come from not just one, but several blazars – colossal cosmic giants with black holes billions of times the size of the Sun at their center.
An ultra-energetic cosmic messenger
Blazars are some of the brightest, most energetic objects in the known Universe. They are a particularly luminous type of quasar, as the relativistic jets emanating from their black holes are shining in our direction.
Neutrinos, meanwhile, are arguably the most reliable carriers of information across the cosmos. For decades, scientists have aimed to uncover their secrets.
Unlike photons, neutrinos are largely undisturbed by matter and electromagnetic fields. This allows them to carry information that would otherwise be degraded, meaning they can provide a wealth of information about their source and the wider cosmos.
Their nickname, “ghost particles”, comes from the fact they have virtually no mass and pass effortlessly through solid matter without leaving a trace. In fact, roughly a hundred trillion are flying through you in any one second.
Despite their ubiquity, they are notoriously difficult to detect. They require precise instruments deep below the Earth’s surface to avoid cosmic interference and background noise.
On February 13, 2023, the most powerful neutrino ever observed was discovered by one of these instruments: a deep-sea neutrino detector called the Kilometer Cubic Neutrino Telescope (KM3NeT).
Though it is still under construction 11,300 feet (3,450 meters) beneath the waves, KM3NeT was able to capture an energy reading of approximately 220 PeV – making the 2023 neutrino 30 times more energetic than the previous record holder.
To better understand this ultra-powerful “ghost particle”, scientists at KM3NeT simulated several events and compared the results with real observations. In a press statement, they compared this process to a forensic investigation of a crime scene.
Investigating the source
Typically, when a high-energy neutrino is discovered, scientists look for an electromagnetic “counterpart”. This is detected as a light emission in the form of radio, optical, X-ray, or gamma-ray waves. If a source is observed, it links the neutrino to a specific single event in a region of the sky.
In the case of the 2023 neutrino, this counterpart was conspicuous by its absence. This doesn’t completely rule out the possibility that the neutrino came from a single source, but it does open the door to other hypotheses.
“There are several possible explanations for the origin of this particle,” Meriem Bendahman, a researcher at INFN Naples and a member of the KM3NeT collaboration, one of the authors of the study, explained in the statement.
“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,” Bendahman continued. “But there is also the possibility that the neutrino originates from a diffuse flux produced by a population of extreme accelerators, such as blazars.”
This “diffuse flux” refers to the fact that the neutrino may have come from many sources, rather than a single point of origin.
To delve deeper, the KM3NeT team chose to simulate a population of blazars using an open-source software called AM3. They fixed parameters to values known from real blazar observations, such as the magnetic field strength and the size of the emission region.
“We modelled a realistic population of blazars with physically motivated parameters,” Bendahman said, “and we found that this population of blazars could explain the origin of this ultra-high-energy event”.
A ‘new window’ into the neutrino universe
Understanding the origins of the ultra-energetic 2023 ghost particle could teach us a great deal about blazars and neutrinos themselves.
“If it turns out to [have] come from cosmic accelerators like blazars,” Bendahman noted, “it would give us new insight into how these objects can emit particles at energies beyond what we previously expected.”
Bendahman expects to learn more in the not-too-distant future. Impressively, the detection was made while only 21 detection lines of KM3NeT were active. That accounts for roughly 10% of its final capacity, when construction is completed by around 2028.
“KM3NeT is still under construction, and we detected this ultra-high-energy neutrino with only a partial configuration,” Bendahman 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.”
This research was published in the Journal of Cosmology and Astroparticle Physics
Fact-checked by Mike McRae
