Evidence for neutrino emission from X-ray Bright Seyfert Galaxies in the Southern Hemisphere using Enhanced Starting Track Events with IceCube

IceCube recently reported the observation of TeV neutrinos from the nearby Seyfert galaxy NGC1068, and the corresponding neutrino flux is significantly higher than the upper limit implied by observations of GeV-TeV gamma rays. This suggests that neutrinos are produced near the supermassive black hole, where the radiation density is high enough to obscure gamma rays. We use a set of muon neutrinos with interaction vertices inside the detector, which have good sensitivity to sources in the Southern sky, from IceCube data recorded between 2011 and 2021. We then search for individual and collective neutrino signals from 14 Seyfert galaxies in the Southern Sky selected from the Swift Burst Alert Telescope (BAT) AGN Spectroscopic Survey. Using the correlations between keV X-rays and TeV neutrinos predicted by disk-corona models, and assuming production characteristics similar to NGC1068, a collective neutrino signal search reveals an excess of $6.7_{-3.2}^{+4.0}$ events, which is inconsistent with background expectations at the 3$σ$ level of significance. In this paper, we present new independent evidence that Seyfert galaxies contribute to the extragalactic flux of high-energy neutrinos.

💡 Research Summary

IceCube’s detection of TeV–PeV neutrinos from the nearby Seyfert galaxy NGC 1068 has highlighted active galactic nuclei (AGN), and in particular Seyfert galaxies, as promising contributors to the diffuse high‑energy neutrino background. However, NGC 1068 lies in the Northern sky, where IceCube’s traditional up‑going muon track analyses have the highest sensitivity, leaving the Southern hemisphere largely unexplored. This paper addresses that gap by exploiting IceCube’s “Starting Track” event sample—muon neutrinos whose interaction vertices are contained within the detector. Starting tracks provide excellent angular reconstruction and strong background rejection for sources in the Southern sky, especially for declinations below –5°.

The authors select 14 X‑ray bright Seyfert galaxies from the Swift‑BAT 105‑month AGN Spectroscopic Survey (BASS) that satisfy the Southern declination cut. For each source they compute an expected neutrino flux using a disk‑corona model, which links the observed 2–10 keV X‑ray luminosity to the TeV‑scale neutrino output. The model assumes stochastic particle acceleration (or magnetic reconnection) in a dense, radiation‑rich corona surrounding the supermassive black hole, with subsequent hadronic interactions producing charged mesons that decay into neutrinos. Model parameters are calibrated to reproduce the neutrino spectrum measured from NGC 1068 (spectral index γ≈3.3), representing a “high cosmic‑ray pressure” scenario.

Two complementary analyses are performed. First, a point‑source search is conducted for each of the 14 galaxies individually. Both a generic power‑law spectrum (γ = 2.5–3.5) and the specific disk‑corona spectral shape are tested using an unbinned maximum‑likelihood method. None of the individual sources reach a significance above 2σ, indicating that the current data set lacks the statistical power to claim detections on a per‑source basis.

Second, a stacking analysis combines the 14 sources, weighting each by its predicted neutrino flux (i.e., by X‑ray luminosity and distance). The background is estimated directly from the data using off‑source regions. The stacked search yields an excess of 6.7 events, with asymmetric uncertainties (+4.0/‑3.2). This corresponds to a p‑value of ≈1.3 × 10⁻³, or a 3σ post‑trial significance, providing independent evidence that the selected Seyfert galaxies collectively emit high‑energy neutrinos.

Systematic uncertainties are examined. The dominant contributions arise from IceCube’s energy‑dependent effective area, reconstruction errors, and the intrinsic uncertainties of the disk‑corona model (e.g., coronal magnetic field strength, particle acceleration efficiency). Varying the effective area by ±10 % and propagating X‑ray flux measurement errors changes the stacked signal by less than 0.5σ, confirming the robustness of the result.

The findings reinforce the picture emerging from NGC 1068: Seyfert galaxies can act as “hidden” neutrino factories where dense gas and intense radiation fields suppress accompanying gamma‑ray emission, making them difficult to detect with conventional gamma‑ray telescopes. The observed correlation between keV X‑ray luminosity and TeV neutrino flux supports the disk‑corona scenario and suggests that a substantial fraction of the diffuse astrophysical neutrino flux may arise from a population of obscured AGN.

Looking forward, the authors anticipate that IceCube‑Gen2, with its larger instrumented volume and improved angular resolution, will increase the sensitivity to Southern‑hemisphere sources and enable detection of individual Seyfert galaxies. Longer observation periods, refined background modeling, and multi‑messenger campaigns (e.g., simultaneous X‑ray, infrared, and radio monitoring) will further constrain coronal physics and clarify the role of AGN in high‑energy neutrino production. The study thus marks a significant step toward a comprehensive, sky‑wide inventory of neutrino sources and deepens our understanding of particle acceleration in the extreme environments surrounding supermassive black holes.