IceCube's measurement of the diffuse astrophysical neutrino flux with six years of high-energy cascade data (arXiv:2001.09520) established a smooth, all-flavor flux but left its physical origin unidentified: the sizable sub-100 TeV component must come from sources opaque to gamma rays to avoid overproducing the Fermi isotropic diffuse gamma-ray background, yet no source class is singled out; the spectral shape could not be distinguished from a single power law at significance; prompt atmospheric neutrinos from charm production remain undetected; and the share of known gamma-transparent populations such as BL Lac blazars is bounded only loosely. This problem asks for a quantitatively defended attribution of the diffuse TeV-PeV neutrino flux to identified astrophysical source populations, with their fractional contributions as a function of energy, the measured spectral form, and a detected or tightly constrained prompt atmospheric component. The central identification question remains open.

Background

The IceCube Neutrino Observatory, a cubic-kilometer Cherenkov detector at the South Pole, has measured a diffuse flux of astrophysical neutrinos spanning TeV to PeV energies. 'Diffuse' here means the unresolved sum over cosmic sources, as opposed to point-source emission; it is measured above two atmospheric backgrounds: 'conventional' neutrinos from the decay of mesons produced by cosmic-ray interactions in the atmosphere, and 'prompt' neutrinos from the decay of charmed hadrons, which are harder in spectrum and still undetected. The six-year cascade analysis (Aartsen et al., Phys. Rev. Lett. 125, 121104 (2020)) used 'cascade' events -- showers from electron- and tau-neutrino interactions with contained vertices, which have good energy resolution -- to characterize the all-flavor flux, and found a single power law (SPL, flux proportional to E−γE^{-\gamma}) to be the best fit, with modestly curved alternatives (cutoff, break, log-parabola) only marginally favored (the log-parabola, hypothesis C, at p-value about 0.06). Because neutrinos are produced alongside gamma rays in hadronic ('cosmic-ray') accelerators via pion decay, the neutrino sky is tied to the gamma-ray sky: any population transparent to gamma rays and intense enough to supply the neutrino flux below roughly 100 TeV would overproduce the isotropic diffuse gamma-ray background (IGRB) measured by the Fermi-LAT space gamma-ray telescope, so the dominant sub-100 TeV sources must be 'gamma-opaque' -- dense environments such as the coronae (hot plasma close to the central black hole) of active galactic nuclei (AGN), jets that remain choked, i.e. buried inside their host galaxy, or star-forming regions that absorb their own gamma rays. For gamma-transparent classes the paper constrained the contribution of BL Lac blazars (active galaxies whose relativistic jets point toward Earth, making them bright gamma-ray emitters) through the ratio YνγY_{\nu\gamma} of neutrino to gamma-ray luminosity (fit value 0.20−0.09+0.120.20^{+0.12}_{-0.09}, upper limit 0.41 at 90% confidence). Prior point-source limits already showed that Fermi-2LAC blazars (the gamma-ray-loud active galaxies in the second Fermi-LAT AGN catalog) supply at most 27% of the flux between about 10 TeV and 2 PeV for spectral index 2.5 (Aartsen et al., Astrophys. J. 835, 45 (2017)). The all-flavor starting-event spectrum disfavors a single power law at about 4.2 sigma (Balagopal V. et al., PoS ICRC2025, 985), and ten-year point-source searches identify NGC 1068 as the first confirmed gamma-opaque neutrino source (Abbasi et al., Science 378, 538 (2022)). What remains missing is a physical identification: which populations actually dominate, in what proportions across energy, with what spectral shape, and with how large a residual prompt atmospheric background.

Problem Statement

Determine the physical origin of the diffuse astrophysical neutrino flux measured by IceCube, as framed by the six-year cascade analysis (arXiv:2001.09520) and its successors: (i) identify which astrophysical source populations produce the all-flavor TeV-PeV flux and with what fractional contributions as a function of energy, in particular the sizable, smooth component below roughly 100 TeV, which must be emitted by sources opaque to gamma rays so as not to overproduce the Fermi-LAT isotropic diffuse gamma-ray background; (ii) establish the spectral form of the flux -- single power law versus curved or broken shapes -- and its physical interpretation; and (iii) detect, or constrain tightly enough to be non-degenerate, the prompt atmospheric neutrino flux from charm production so that the residual background under the astrophysical component is known; with the contribution of known gamma-transparent classes such as BL Lac blazars bounded. The identification sought is a quantitatively defended attribution of the diffuse flux to identified source classes across the observed energy range -- with the spectral form of the flux established, the prompt atmospheric component detected or constrained tightly enough to be non-degenerate, and the attribution consistent with the neutrino data, the gamma-ray backgrounds, source-count and point-source limits, and multi-messenger observations -- such that the dominant sub-100 TeV gamma-opaque component is identified and its share, together with the shares of all other significant populations, is quantified.

References

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