A Seyfert galaxy as a hidden counterpart to a neutrino-associated blazar

The origin and production mechanisms of high-energy astrophysical neutrinos remain open questions in multimessenger astronomy. Previous studies have hinted at a possible linear correlation between the hard X-ray and high-energy neutrino emission in active galactic nuclei. New \textit{NuSTAR} observations, first presented here, reveal that blazar PKS 1424+240, located within a prominent IceCube neutrino hotspot, is far fainter in hard X-rays than expected from this trend. Motivated by this apparent ambiguity, we identify the nearby Seyfert galaxy NGC 5610, also coincident with the hotspot, whose unabsorbed hard X-ray flux exceeds that of PKS 1424+240 by about an order of magnitude. When the local IceCube neutrino flux is apportioned between the two AGN in proportion to their hard X-ray emission, both align with the previously suggested X-ray-neutrino correlation. This suggests that certain IceCube hotspots may be unresolved blends of multiple AGN, and supports a multimessenger scenario in which high-energy neutrinos and hard X-rays originate from the same hadronic interactions, with the X-ray emission produced through cascade reprocessing.

💡 Research Summary

The paper investigates the long‑standing puzzle of the origin of high‑energy astrophysical neutrinos (0.3–100 TeV) by testing a proposed linear relationship between hard X‑ray luminosity (15–55 keV) and neutrino luminosity in active galactic nuclei (AGN). Earlier work (reference 9) identified a tight correlation across a small sample of four radio‑quiet Seyfert galaxies and two radio‑loud blazars, suggesting that both neutrinos and hard X‑rays are produced in the same compact region near the supermassive black hole, where dense ultraviolet/X‑ray photon fields enable efficient proton‑photon (pγ) interactions. The resulting pions decay into neutrinos, while the accompanying high‑energy photons are absorbed (γγ → e⁺e⁻) and cascade down to the hard X‑ray band, providing a multimessenger signature.

To test whether this correlation extends to the blazar PKS 1424+240, which has been associated with an IceCube neutrino hotspot at a pre‑trial significance of ~3.7 σ, the authors obtained new NuSTAR observations. The NuSTAR spectrum is well described by a power‑law with photon index Γ ≈ 1.70 and a 15–55 keV flux of (4.9 ± 1.0) × 10⁻¹³ erg s⁻¹ cm⁻², indicating that the blazar is unusually faint in hard X‑rays. Using the IceCube ten‑year Pass 2 track dataset, the inferred neutrino flux for the hotspot is F_ν ≈ 5.1 × 10⁻¹¹ erg s⁻¹ cm⁻², which is roughly two orders of magnitude higher than expected from the X‑ray–neutrino trend. This discrepancy could imply that the neutrino emission does not originate from the blazar’s jet, or that the correlation fails for blazars.

The authors then note that the Seyfert galaxy NGC 5610 lies only ~1° from PKS 1424+240 and falls within IceCube’s angular resolution (≈1°). NGC 5610, at redshift z ≈ 0.017, exhibits a Swift/BAT 0.3–150 keV flux of 1.92 × 10⁻¹¹ erg s⁻¹ cm⁻² and a hard X‑ray (15–55 keV) unabsorbed flux of (4.7 ± 0.4) × 10⁻¹² erg s⁻¹ cm⁻²—about ten times brighter than the blazar. Assuming that the total IceCube neutrino excess is shared between the two AGN in proportion to their hard X‑ray fluxes, the authors allocate ≈4.8 × 10⁻¹² erg s⁻¹ cm⁻² to PKS 1424+240 and ≈4.6 × 10⁻¹¹ erg s⁻¹ cm⁻² to NGC 5610. When these revised neutrino luminosities are plotted against the corresponding X‑ray luminosities, both sources fall on the previously established correlation line (log L_X = 0.81 log L_ν + 8.06).

Statistical analysis of the expanded sample (now eight AGN: three blazars and five Seyferts) yields a Pearson correlation coefficient R ≈ 0.92 and a Spearman coefficient R ≈ 0.86, confirming that the linear trend remains robust. Partial correlation coefficients that control for redshift (R_p ≈ 0.51 for Pearson, R_p ≈ 0.69 for Spearman) remain positive, indicating that the relationship is not solely driven by distance‑related selection effects. The authors acknowledge that flux thresholds in both X‑ray and neutrino surveys can induce apparent luminosity correlations, but their partial‑correlation analysis suggests a genuine physical link.

The physical interpretation reinforced by the results is that high‑energy neutrinos are predominantly produced in the dense photon environments of AGN coronae, where pγ interactions have high optical depth. The cascade of secondary photons down to the hard X‑ray band provides a natural tracer of the target photon field. In blazars, where external photon fields are weaker, the same mechanism can operate but with reduced efficiency, explaining why blazars generally appear X‑ray bright relative to the broader VLBI population yet still fall on the correlation when their X‑ray contribution is properly accounted for.

A key implication of the study is that IceCube point‑source hotspots may often be unresolved blends of multiple AGN, especially in regions where several X‑ray‑bright sources lie within the ~0.1°–2° angular uncertainty of track events. This “source confusion” could affect the interpretation of individual neutrino associations and underscores the necessity of high‑resolution, multi‑wavelength follow‑up. The authors propose that future facilities such as IceCube‑Gen2, the Cherenkov Telescope Array (CTA), and next‑generation X‑ray observatories (eROSITA, Athena) will enable more precise localization and spectral characterization, allowing the X‑ray–neutrino correlation to become a powerful diagnostic of particle acceleration and hadronic processes in AGN cores.

In summary, the paper (1) identifies NGC 5610 as a previously unrecognized contributor to the IceCube neutrino excess near PKS 1424+240, (2) expands the hard X‑ray–neutrino luminosity sample from six to eight AGN, strengthening the observed correlation, and (3) demonstrates that indirect multimessenger inference can reveal neutrino sources even when direct electromagnetic counterparts are ambiguous. The findings support a unified picture in which pγ interactions in dense UV/X‑ray fields near supermassive black holes generate both high‑energy neutrinos and hard X‑ray emission across both radio‑quiet Seyferts and radio‑loud blazars.