Evidence for Neutrino Emission from X-ray Bright Active Galactic Nuclei with IceCube

TL;DR

This study extends IceCube’s northern-sky neutrino analysis to 13.1 years to test neutrino emission from NGC 1068 and a targeted sample of X-ray-bright AGN. Using an unbinned likelihood framework with KDE-derived PDFs for IC79 and IC86, the authors perform a sky survey and two candidate-source tests: a catalog of 110 gamma-ray emitters and a list of 47 X-ray-bright AGN, plus binomial tests for ensembles. The all-sky search identifies a hotspot compatible with NGC 1068 (local $5\sigma$, global $1.4\sigma$); in the gamma-ray emitter catalog, NGC 1068 yields global $\sim4\sigma$ with $n_s\approx102$ and $\gamma\approx3.4$, while the X-ray-bright AGN analysis reveals a 11-source binomial excess with post-trial $\sim3.3\sigma$, led by NGC 7469 with a hard spectrum. Collectively, the results strengthen the case that X-ray-bright AGN cores contribute to the diffuse extragalactic neutrino flux, consistent with neutrino production in AGN coronae, while highlighting source-to-source diversity and the need for time-dependent studies and next-generation detectors.

Abstract

Recently, IceCube reported neutrino emission from the Seyfert galaxy NGC 1068. Using 13.1 years of IceCube data, we present a follow-up search for neutrino sources in the northern sky. NGC 1068 remains the most significant neutrino source among 110 preselected gamma-ray emitters while also being spatially compatible with the most significant location in the northern sky. Its energy spectrum is characterized by an unbroken power-law with spectral index $γ= 3.4 \pm 0.2$. Consistent with previous results, the observed neutrino flux exceeds its gamma-ray counterpart by at least two orders of magnitude. Motivated by this disparity and the high X-ray luminosity of the source, we selected 47 X-ray bright Seyfert galaxies from the Swift/BAT spectroscopic survey that were not included in the list of gamma-ray emitters. When testing this collection for neutrino emission, we observe a 3.3$σ$ excess from an ensemble of 11 sources, with NGC 1068 excluded from the sample. Our results strengthen the evidence that X-ray bright cores of active galactic nuclei are neutrino emitters.

Figures (13)

• Figure 1: Agreement between the distributions of the analysis observables for experimental data (dots) and simulations (solid lines) for both IC79 (pink) and IC86 (blue). From left to right, we show the reconstructed muon energy, $E_{\mu}$, which ranges from 100 GeV to a few PeV, the sine of the reconstructed declination, sin($\delta_{\mu}$), and the reconstruction quality estimator on the muon track direction $\sigma_{\mu}$. In the lower panels, we display the ratios between the experimental data and the simulations for both detector configurations. The simulations show the sum of the atmospheric and the diffuse astrophysical neutrino flux components. The atmospheric neutrino flux assumes the GST model GST:2013FrPhy for the primary cosmic-ray spectrum and Sybill2.3c as interaction model Sybill2.3c. The diffuse astrophysical component assumes a single power-law with spectral index $\gamma_{\mathrm{astro}} = 2.37$ and normalization $\phi_{\mathrm{astro}} = 1.44\times10^{-18}~\mathrm{GeV}^{-1}~\mathrm{cm}^{-2}~\mathrm{s}^{-1}~\mathrm{sr}^{-1}$IceCube:2022NTDiffuse.
• Figure 2: On the left, we show the $p$-value map of the northern sky, obtained under the hypothesis of a free spectral index. The map is shown in equatorial coordinates on a Hammer-Aitoff projection. The color bar represents the local significance of each pixel in the sky, and we highlight the location of the strongest emission, found to be spatially compatible with the Seyfert II galaxy NGC 1068. On the right, we show a zoom-in of the hottest spot. The white cross represents the best-fit location, the solid (dashed) line represents the 68% (95%) uncertainty contour of the excess, and the orange dot and circle represent, respectively, the source location and its optical size Paturel:2003AA.
• Figure 3: Left: Profile likelihood scan for the flux parameters of NGC 1068. We highlight in white the best-fit result (cross) and the 68% and 95% uncertainty contours (solid and dashed lines), derived from Wilks' theorem. The color scale indicates the log-likelihood ratio difference between any point in the parameter space and the overall best-fitting point. Right: best-fit and 68% contour comparison between this work (blue), IceCube:2022Science (yellow), and IceCube:2020PhRvL (light blue) -- the first IceCube analysis that found NGC 1068 as the most significant source in a list, with a global significance of 2.9$\sigma$. All contours include only statistical uncertainties.
• Figure 4: Visualization of the binomial test excess for the list of 110 gamma-ray emitters (left) and 47 X-ray-bright AGN (right). In the upper panels, we show the local $p$-values for each source, ordered from the lowest to the highest (black points), the background expectation (dashed black line), and its 1$\sigma$, 2$\sigma$, and 3$\sigma$ Poissonian uncertainties (shaded blue bands). In the lower panels, we report the binomial probability for each subset of sources. The most significant excess is highlighted by dotted red lines. To preserve readability, the plot is truncated at the point where sources reach a p-value of 1.0, beyond which the binomial probability is also equal to 1.0.
• Figure 5: Best-fit neutrino all-flavor fluxes with their 1$\sigma$ uncertainty and constrained in the 95% C.L. energy range of the top 4 sources in the list of X-ray bright AGN. The yellow band represents the most recent measurement of the diffuse astrophysical neutrino flux. The energy range is constrained at the 90% C.L., while the flux normalization is shown with a 68% C.L. abbasi2025:spectralbreak.