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KM3NeT collaboration, MessMapp Group et al.

The KM3NeT experiment reported the detection of an ultra-high-energy neutrino with an energy estimate of ~ 220 PeV, the most energetic yet observed. The neutrino arrival direction has a 99% confidence region of 3° radius centred at RA 94.3°, Dec -7.8° (J2000). High-energy astrophysical neutrinos are a crucial messenger for understanding hadronic acceleration processes in the Universe and for identifying the origin of ultra-high-energy cosmic rays. Among the most powerful cosmic accelerators, blazars are proposed as promising neutrino sources. A sample of seventeen candidate blazars located in this region is selected through their multiwavelength properties, and studied using archival data and dedicated observations. One of the candidate counterparts exhibits a radio flare coinciding with the neutrino arrival time, with a pre-trial chance probability of 0.26%. Another candidate counterpart exhibits a rising trend in the X-ray flux in a one-year window around the neutrino arrival time. A third candidate undergoes a gamma-ray flare during the same period. While none of these candidates can conclusively be linked to the neutrino, the implications of a possible blazar origin for the KM3NeT event are discussed.

KM3NeT collaboration et al.

The detection of cosmic neutrinos with energies above a teraelectronvolt (TeV) offers a unique exploration into astrophysical phenomena. Electrically neutral and interacting only by means of the weak interaction, neutrinos are not deflected by magnetic fields and are rarely absorbed by interstellar matter: their direction indicates that their cosmic origin might be from the farthest reaches of the Universe. High-energy neutrinos can be produced when ultra-relativistic cosmic-ray protons or nuclei interact with other matter or photons, and their observation could be a signature of these processes. Here we report an exceptionally high-energy event observed by KM3NeT, the deep-sea neutrino telescope in the Mediterranean Sea, which we associate with a cosmic neutrino detection. We detect a muon with an estimated energy of 120(+110,-60) petaelectronvolts (PeV). In light of its enormous energy and near-horizontal direction, the muon most probably originated from the interaction of a neutrino of even higher energy in the vicinity of the detector. The cosmic neutrino energy spectrum measured up to now falls steeply with energy. However, the energy of this event is much larger than that of any neutrino detected so far. This suggests that the neutrino may have originated in a different cosmic accelerator than the lower-energy neutrinos, or this may be the first detection of a cosmogenic neutrino8, resulting from the interactions of ultra-high-energy cosmic rays with background photons in the Universe.

Clairfontaine et al.

Recent observations are shedding light on the important role that active galactic nuclei play in the production of high-energy neutrinos. In this study, we focus on one object, 5BZB J0630-2406, which is among the blazars recently proposed as associated with neutrino emission during the first 7 yr of IceCube observations. Modeling the quasi-simultaneous, broadband spectral energy distribution, we explore various scenarios from purely leptonic to leptohadronic models, testing the inclusion of external photon fields. This theoretical study provides a complementary testing ground for the proposed neutrino-blazar association. Despite being historically classified as a BL Lac, our study shows that 5BZB J0630-2406 belongs to the relatively rare subclass of high-power flat-spectrum radio quasars. Our results indicate that interactions between protons and external radiation fields can produce a neutrino flux that is within the reach of the IceCube detector. Furthermore, the spectral shape of the X-ray emission suggests the imprint of hadronic processes related to very energetic protons.

Buson et al. 2023

Identifying the astrophysical sources responsible for the high-energy cosmic neutrinos has been a longstanding challenge. In a previous work, we report evidence for a spatial correlation between blazars from the 5th Roma-BZCat catalog and neutrino data of the highest detectable energies, i.e. > 0.1 PeV, collected by the IceCube Observatory in the southern celestial hemisphere. The statistical significance is found at the level of 2 x 10^{-6} post-trial. In this work we test whether a similar correlation exists in the northern hemisphere, were IceCube is mostly sensitive to < 0.1 PeV energies. We find a consistent correlation between blazars and northern neutrino data at the pre-trial p-value of 5.12 x 10^{-4}, and a post-trial chance probability of 6.79 x 10^{-3}. Combining the post-trial probabilities observed for the southern and northern experiments yields a global post-trial chance probability of 2.59 x 10^{-7} for the genuineness of such correlation. This implies that the spatial correlation is highly unlikely to arise by chance. Our studies push forward an all-sky subset of 52 objects as highly likely PeVatron extragalactic accelerators.

Buson et al. 2022, ApJL , 933, 4

Neutrinos are the most elusive particles in the universe, capable of traveling nearly unimpeded across it. Despite the vast amount of data collected, a long-standing and unsolved issue is still the association of high-energy neutrinos with the astrophysical sources that originate them. Among the candidate sources of neutrinos, there are blazars, a class of extragalactic sources powered by supermassive black holes that feed highly relativistic jets, pointed toward Earth. Previous studies appear controversial, with several efforts claiming a tentative link between high-energy neutrino events and individual blazars, and others putting into question such relation. In this work, we show that blazars are unambiguously associated with high-energy astrophysical neutrinos at an unprecedented level of confidence, i.e., a chance probability of 2 × 10^{−6}. Our statistical analysis provides the observational evidence that blazars are astrophysical neutrino factories and hence, extragalactic cosmic-ray accelerators.

Buson et al. 2022, ApJL, 934, 2

Amendement to the original article.

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Baldini et al. 2021, ApJS, 256, 13