Neutrinos from extreme astrophysical sources

Abstract

In this paper I review recent results on high-energy neutrino astronomy and what they can reveal about some of the most extreme cosmic accelerators. I discuss recent measurements of the diffuse TeV-PeV cosmic neutrino spectrum by the IceCube observatory and the current flux limits in the ultra-high-energy regime, contextualizing the recent detection of an ultra-high-energy neutrino by the KM3NeT observatory. I review the recent emergence of a TeV signal from nearby Seyfert galaxies such as NGC 1068, the potential of $γ$-ray blazars as neutrino sources above the PeV regime, and the current status of tidal disruption events and other transient classes as possible neutrino sources. For each of these topics, I discuss ongoing developments in source models and their current limitations. I argue for the indispensable role of next-generation multi-messenger facilities, such as IceCube-Gen2, in solidifying current source associations, probing the ultra-high-energy regime, and resolving vast transient populations that remain unidentified with current statistics.

Figures (3)

• Figure 1: Diffuse flux of all-flavor cosmic neutrinos between a TeV and a few PeV IceCube:2025tgp and current limits on the UHE flux by IceCube IceCubeCollaborationSS:2025jbi and the Pierre Auger Observatory PierreAuger:2023pjg. The blue data point is derived from KM3NeT's detection of the $>100$ PeV event KM3-230213A, taking into consideration the upper limits by other experiments KM3NeT:2025ccp. The three curves shaded underneath represent model predictions for the diffuse flux from X-ray-bright Seyferts Padovani:2024tgx, $\gamma$-ray blazars Padovani:2015mba and cosmogenic neutrinos produced by UHE cosmic rays from a population of jetted AGN Rodrigues:2020pli. The model predictions are constrained by stacking limits on the population of X-ray-bright Seyferts IceCube:2024dou and $\gamma$-ray-bright blazars IceCube:2016qvd. The sensitivity of the future IceCube-Gen2 in the UHE range, enhanced by the planned in-ice radio array, is shown in bright green IceCube-Gen2:2020qha.
• Figure 2: Measurements of NGC 1068 in GeV $\gamma$-rays by the Fermi LAT (green data points) and best-fit neutrino flux by IceCube (blue band). The colored curves represent the neutrino spectrum (right-hand side) and the accompanying $\gamma$-ray spectrum (shaded underneath) predicted by different hadronic source models: Murase:2019vdl, Eichmann:2022lxh, Fiorillo:2023dts, Das:2024vug, and Karavola:2024uui. Since neutrinos should originate in an optically thick region, a less compact region must be responsible for the GeV $\gamma$-ray emission, in the case shown as a dotted curve, the starburst region surrounding the AGN core Eichmann:2022lxh.
• Figure 3: Blazar TXS 0506+056 was the first source to be associated to a high-energy neutrino event in 2017, in temporal coincidence with a half-year-long multi-wavelength flare IceCube:2018dnn. The black data points show the quasi-simultaneous multi-wavelength measurements IceCube:2018dnnKeivani:2018rnh. The colored curves show the multi-wavelength and neutrino predictions from three source models by Gao:2018mnu, Keivani:2018rnh, and Rodrigues:2025cpm. As we can see, all three models can roughly describe the multi-wavelength emission, but predict neutrino energies differing by more than three orders of magnitude. The model predictions are consistent with the energy deposited by the 2017 event (vertical red line) and with the uncertainty limits on the point source signal, at the 68% confidence level Rodrigues:2024fhu, based on the publicly available ten-year point source data IceCube:2019cia. The sensitivity of IceCube-Gen2 in the UHE range IceCube-Gen2:2020qha, in light green, shows that future experiments will be crucial to test source models.