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

TL;DR

This paper assesses whether AGN can explain IceCube's diffuse high-energy neutrino flux by performing a population-level analysis of X-ray-bright and gamma-ray-bright AGN using 10 years of IceCube data. It implements three flux-scaling hypotheses (X-ray, gamma-ray, geometric) across Swift-BAT and 4LAC-DR3 catalogs, allowing for per-source flux scatter and applying completeness factors. The principal findings are that gamma-ray-bright blazars contribute at most ~15% of the diffuse flux, gamma-ray-bright non-blazar AGN show no strong evidence, while X-ray-bright, non-blazar AGN—driven by nearby Seyferts like NGC 1068—exhibit significant neutrino signals and may account for a substantial portion (11%–100%) of IceCube’s diffuse flux. A robust 4.2sigma correlation with the Swift-BAT AGN population (dominated by NGC 1068) further supports the role of gamma-ray-obscured AGN as notable high-energy neutrino sources, with implications for future detectors and source-search strategies.

Abstract

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.

Figures (8)

• Figure 1: The distribution of our test statistic (TS) at 196,608 points evenly spaced across the sky, adopting a spectral index of 2.5. This result follows a gaussian distribution and reveals no evidence of any neutrino source population.
• Figure 2: Constraints on the total neutrino flux from X-ray-bright, non-blazar AGN (per flavor and evaluated at $E_{\nu} =30\,{\rm TeV}$), and on the source-to-source variation in this flux, $\delta$. We also plot the test statistic (TS) profile as a function of each of these parameters. These fluxes include a completeness factor that accounts for the fraction of the source population that is not contained in the 70-month Swift-BAT catalog (see Table \ref{['completeness_factors_all']}). In the top and middle rows, we show the results for the hard and soft X-ray scaling hypotheses, respectively. In the bottom row, we show the results for the geometric scaling hypothesis. In the left and right frames, we adopt a neutrino spectral index of $\alpha=2.5$ or 3.0. The vertical gray band denotes the total diffuse flux measured by IceCube icecube_diffuse_recent.
• Figure 3: As in Fig. \ref{['swift_full']}, but excluding NGC 1068.
• Figure 4: Constraints on the total neutrino flux from gamma-ray-bright, blazar AGN (per flavor and evaluated at $E_{\nu} =30\,{\rm TeV}$), and on the source-to-source variation in this flux, $\delta$. These fluxes include a modest completeness factor that accounts for the fraction of the source population that is not contained in the Fermi 4LAC-DR3 catalog (see Table \ref{['completeness_factors_all']}). We also plot the test statistic (TS) profile as a function of each of these parameters. In the top rows, we show the results for the gamma-ray scaling hypotheses. In the bottom row, we show the results for the geometric scaling hypothesis. In the left and right frames, we have adopted a neutrino spectral index of $\alpha=2.5$ and 3.0. The vertical gray band denotes the total diffuse flux measured by IceCube icecube_diffuse_recent.
• Figure 5: As in Fig. \ref{['4LAC_all_blazars_fig']}, but restricting our sample to non-variable gamma-ray-bright AGN.