The IceCube Neutrino Observatory and the KM3NeT Astroparticle Research with Cosmics in the Abyss (ARCA) telescope are neutrino telescopes that search for the sources of astrophysical neutrinos, cosmic messengers that can reveal hidden parts of the universe. Recently, the KM3NeT Collaboration reported the detection of the highest energy neutrino candidate ever discovered, at 220 PeV (petaelectronvolt, or a million billion eV). Dubbed KM3-230213A, its energy is equivalent to a ping-pong ball dropped from a desk but concentrated in one neutrino instead of across the 1023 atoms in a ball.
Scientists are trying to pinpoint the origin of KM3-230213A in order to understand the physical processes that produced it. In addition, studying these ultra-high-energy (UHE) events could help uncover new particles, forces, or symmetries that exist beyond the ones we currently know.
In a study submitted to Physical Review Letters, the IceCube Collaboration searched for high-energy neutrinos in the direction of KM3-230213A using 15 years of IceCube data. They considered three temporal hypotheses: steady-state, flaring, and various time-windows centered at KM3NeT’s detection time. No evidence for neutrino emission was found for any of the studies performed and upper limits were set on the neutrino flux from a point source in the direction of KM3-230213A.
Thus far, there has been much speculation about its origin, particularly because IceCube has yet to observe a neutrino at such a high energy, despite being a larger and longer-running experiment. Meanwhile, the KM3NeT/ARCA telescope detected KM3-230213A despite being only 10% complete.
“One explanation is that the source that produced this ultra-high-energy event was a ‘transient’ source, meaning the conditions that accelerated the cosmic ray that produced this event were only active for a certain moment in time,” explains Sarah Mancina, a postdoctoral researcher at Università degli Studi di Padova. “However, some models predict that if a source is producing an event like this, it should also produce more neutrinos at lower energies that may have been observed by IceCube.”
Mancina coled the study along with University of Wisconsin–Madison PhD student Alicia Mand and recent PhD graduate Riya Shah, who conducted the study while at Drexel University.
For the follow-up analyses, Mand and Shah looked for neutrino sources both in the direction of KM3-230213A and around the time it was observed. They both used a combination of tracks and cascades, two different neutrino event types that IceCube is sensitive to, to perform their analyses.
For a time-integrated search, she combed through IceCube data to find any high-energy neutrinos coincident with the direction of KM3-230213A. A similar method was used for the time-dependent analysis, except specific time windows (1000 seconds, 10,000 seconds, and 10,000,000 seconds) were used in the search.
Shah’s analysis checked for any neutrino sources in the direction of the KM3NeT event that might be flaring over the 15 years that IceCube has been taking data. She found no significant flaring point source within three degrees of KM3-230213A location.
“Over the past 15 years, IceCube has not seen an event at this high of an energy,” says Shah. “The results of our tests show that the origin of this event still remains a mystery!”
Mancina works on and maintains IceCube’s gamma-ray follow-up (GFU)-cluster alert stream, which searches for clusters of events in real time to identify candidate neutrino flares. These alerts are then privately shared with gamma-ray experiments. The GFU-cluster alert stream runs in two modes: 1) an all-sky mode, which scans all the regions surrounding the incoming neutrino track-like events and 2) a source list mode that monitors known blazars, supermassive black holes with jets that face the Earth. When Mancina checked the GFU-cluster alerts for activity at the time of the event, she did not find any alerts from that direction.
“The report made by KM3NeT and IceCube’s lack of detection of a high-energy counterpart further motivates the need for UHE neutrino detectors in order to understand the sources of UHE neutrinos,” says Mand. “I am excited to see the future of the field now that we may be starting to get the first glimpses of the UHE universe.”
+ info “IceCube neutrino point-source searches in the direction of the KM3NeT ultra-high-energy event,” IceCube Collaboration: R. Abbasi et al. Submitted to Physical Review Letters. arXiv