Evaluating the Contribution of Active Galactic Nuclei to the Diffuse High-Energy Neutrino Flux

The detection of high-energy neutrinos from NGC 1068 and TXS-0506+56 suggests that active galactic nuclei (AGN) may contribute significantly to the the diffuse neutrino flux measured by IceCube. Using 10 years of publicly available IceCube data, we performed a systematic population analysis of X-ray-bright and gamma-ray-bright AGN to evaluate the extent to which this diffuse flux could originate from these sources. We find that gamma-ray-bright blazars can account for no more than 16% of IceCube’s total diffuse flux. Although we find no evidence of neutrino emission from gamma-ray-bright, non-blazar AGN, we cannot exclude the possibility that these sources contribute significantly to the diffuse flux. In contrast, we report (pre-trials) evidence of neutrino emission from several nearby, X-ray-bright, Seyfert-type AGN, including \mbox{NGC 1068} ($4.9σ$), SWIFT J1041.4-1740 ($2.6σ$), SWIFT J0202.4+6824A/B ($2.6σ$), SWIFT J0744.0+2914 (2.6$σ$), NGC 4151 ($2.5σ$), and NGC 3079 ($2.5σ$). Although not fully conclusive, these results suggest that IceCube may be detecting neutrinos from a larger population of Seyfert galaxies. The fact that these sources are not gamma-ray bright indicates that their neutrino production must be taking place in optically thick environments, such as in the coronae surrounding these galaxies’ supermassive black holes. We also identify a $4.2σ$ correlation between the neutrinos detected by IceCube and members of the Swift-BAT catalog of X-ray-bright AGN, although this correlation is dominated by NGC 1068. We estimate that this class of sources contributes between 11.2% and the entirety of IceCube’s total diffuse neutrino flux. These results strengthen the emerging case for the prevalence of gamma-ray-obscured AGN as significant sources of high-energy neutrinos.

💡 Research Summary

The authors present a comprehensive population study of active galactic nuclei (AGN) using ten years of publicly released IceCube muon‑track data (2008‑2018). Their goal is to quantify how much of the diffuse high‑energy neutrino flux measured by IceCube can be attributed to different AGN subclasses. They consider two main source samples: (i) 732 non‑blazar, X‑ray‑bright Seyfert‑type AGN drawn from the 70‑month Swift‑BAT hard X‑ray survey, and (ii) γ‑ray‑bright AGN from the Fermi‑LAT 4LAC‑DR3 catalog, comprising 3 339 blazars and 64 non‑blazar AGN. For each sample they test three phenomenological scaling hypotheses: (1) X‑ray scaling, where the neutrino flux of a source is proportional to its intrinsic X‑ray flux (soft and hard bands treated separately); (2) γ‑ray scaling, where the neutrino flux follows the observed γ‑ray flux, representing a hadronic γ‑ray origin; and (3) geometric scaling, where all sources are assumed equally luminous in neutrinos, giving a flux ∝ 1/D_L².

The statistical framework is a stacked likelihood analysis. For each IceCube event they construct signal and background probability density functions (PDFs) based on the event’s reconstructed direction, angular uncertainty, and energy. The expected number of signal events from a source is obtained by folding the model neutrino flux with the muon‑effective area (derived from the published neutrino effective area). To allow for source‑to‑source variability they adopt a log‑normal distribution for the true flux around the model prediction, with a free width parameter δ (0 ≤ δ ≤ 2.5). The test statistic (TS) is defined as twice the log‑likelihood ratio between the best‑fit model and the null hypothesis (no signal).

Key findings:

1. γ‑ray‑bright blazars – The stacked analysis yields no significant excess. The authors place a 95 % confidence upper limit that blazars contribute ≤ 16 % of the total IceCube diffuse neutrino flux, tightening previous constraints.
2. γ‑ray‑bright non‑blazar AGN – No significant signal is found, but the limits are not stringent enough to rule out a substantial contribution.
3. X‑ray‑bright non‑blazar AGN (Swift‑BAT Seyferts) – A clear excess is observed. For the hard‑X‑ray scaling hypothesis with a neutrino spectral index α = 3.0, the TS corresponds to a pre‑trial significance of 4.18 σ (soft‑X‑ray scaling gives 3.98 σ). The geometric scaling hypothesis yields a lower, but still notable, 3.15 σ. Individual sources show pre‑trial significances of 4.9 σ (NGC 1068), 2.6 σ (SWIFT J1041.4‑1740, SWIFT J0202.4+6824A/B, SWIFT J0744.0+2914), 2.5 σ (NGC 4151, NGC 3079). Removing NGC 1068 does not dramatically weaken the overall signal, indicating that the broader Swift‑BAT population is contributing.
4. Correlation with the Swift‑BAT catalog – An independent stacking test finds a 4.2 σ correlation between IceCube events and the full Swift‑BAT AGN list, dominated by NGC 1068 but still significant without it.

By incorporating completeness corrections (accounting for AGN missing from the catalogs), the authors estimate that X‑ray‑bright, non‑blazar AGN could account for anywhere between 11.2 % and 100 % (the entire diffuse flux) at the 2 σ confidence level. This wide range reflects uncertainties in the true source population and the degree of source‑to‑source variability.

The results have important astrophysical implications. The lack of γ‑ray counterparts for the neutrino‑bright Seyferts suggests that neutrino production occurs in optically thick environments, such as the hot coronae surrounding supermassive black holes, where γ‑rays are attenuated but hadronic interactions can still generate high‑energy neutrinos. This supports the emerging picture that “hidden” or γ‑ray‑obscured AGN are a major contributor to the high‑energy neutrino sky, complementing the already established blazar source TXS 0506+56 and the Galactic plane component.

The paper concludes by emphasizing the need for deeper X‑ray surveys, improved γ‑ray sensitivity, and future IceCube upgrades (IceCube‑Gen2) to resolve the contributions of individual Seyfert galaxies and to test the corona‑origin hypothesis more rigorously.