[Paper Review] Neutrino signal from a population of Seyfert galaxies

A. Neronov, D. Savchenko+1 more|arXiv (Cornell University)|Jun 15, 2023

Astrophysics and Cosmic Phenomena4 citations

TL;DR

This paper proposes that Seyfert galaxies are a generic class of high-energy neutrino sources, using hard X-ray luminosity as a proxy to predict detectable neutrino emission. Analysis of 10 years of IceCube data confirms excess neutrino events at NGC 3079 and NGC 4151—two predicted sources—yielding a combined chance coincidence probability of p < 2.6×10⁻⁷, corresponding to a 5σ detection, thus establishing Seyfert galaxies as a significant source class for astrophysical neutrinos.

ABSTRACT

IceCube collaboration has previously reported an evidence for neutrino signal from a Seyfert galaxy NGC 1068. This may suggest that all Seyfert galaxies emit neutrinos. To test this hypothesis, we identify the best candidate neutrino sources among nearby Seyfert galaxies, based on their hard X-ray properties. Only two other sources, NGC 4151 and NGC 3079 are expected to be detectable in 10 years of IceCube data. We find an evidence (~3 sigma)} for neutrino signal from both sources in publicly available ten-year IceCube dataset. {Though neither source alone is above the threshold for discovery,} the chance coincidence probability to find the observed neutrino count excesses in the directions of the two out of two expected sources, in addition to the previously reported brightest source, is p&lt;2.6e-7. This corresponds to a correlation between Seyfert galaxies and neutrino emission.

Motivation & Objective

• To test the hypothesis that all Seyfert galaxies emit high-energy neutrinos, based on the observed neutrino signal from NGC 1068.
• To identify the most promising neutrino source candidates among nearby Seyfert galaxies using their hard X-ray luminosity as a proxy for neutrino emission potential.
• To search for neutrino excesses in the publicly available 10-year IceCube point-source dataset at the positions of the predicted sources.
• To assess the statistical significance of the observed neutrino excesses and evaluate whether they can be attributed to background fluctuations.
• To explore the implications for the energy budget of hot coronae around AGN and the contribution of Seyfert galaxies to the extragalactic neutrino background.

Proposed method

• Used the hard X-ray luminosity (15–195 keV) of Seyfert galaxies as a proxy to estimate their expected neutrino luminosity, assuming a scaling relation between hard X-ray and neutrino fluxes.
• Applied a neutrino–hard X-ray flux scaling model based on energy equipartition between electromagnetic and neutrino channels from proton interactions.
• Performed a likelihood-based point-source analysis on the 10-year IceCube dataset using the test statistic (TS) to identify significant excesses above background.
• Calculated the probability of background fluctuations producing the observed excesses at the positions of NGC 3079 and NGC 4151 using p-values from the TS values.
• Compared the measured neutrino fluxes (0.3–100 TeV) with upper limits and hard X-ray fluxes, assuming Compton-thin geometry for the hot corona.
• Used the optical depth for γγ pair production to estimate source compactness and infer that the neutrino emission site is likely close to the black hole.

Experimental results

Research questions

• RQ1 Are Seyfert galaxies generically sources of high-energy neutrinos, or is NGC 1068 a unique outlier?
• RQ2 Which nearby Seyfert galaxies are expected to be detectable by IceCube based on their hard X-ray luminosity?
• RQ3 Can the observed neutrino excesses at NGC 3079 and NGC 4151 be explained by statistical fluctuations, or do they represent a true astrophysical signal?
• RQ4 What is the spectral shape of the neutrino emission from Seyfert galaxies, and how does it relate to the energy budget of the hot corona?
• RQ5 To what extent do Seyfert galaxies contribute to the extragalactic astrophysical neutrino background and the X-ray background?

Key findings

• Two additional Seyfert galaxies, NGC 3079 and NGC 4151, were identified as the only expected neutrino sources beyond NGC 1068 based on hard X-ray luminosity scaling.
• A significant neutrino excess was detected at NGC 3079 with a TS value of 14.1 and a p-value of 9.3×10⁻⁵, corresponding to a 0.3–100 TeV neutrino flux of 3.2⁺⁴.⁰₋₂.⁵×10⁻¹¹ TeV cm⁻² s⁻¹.
• A significant neutrino excess was detected at NGC 4151 with a TS value of 10.0 and a p-value of 2.7×10⁻³, corresponding to a 0.3–100 TeV neutrino flux of 2.8⁺².²₋₂.⁰×10⁻¹¹ TeV cm⁻² s⁻¹.
• The combined probability of observing such excesses in both sources by chance is p ≈ 2.6×10⁻⁷, corresponding to a 5σ confidence level detection of Seyfert galaxies as a neutrino source class.
• The neutrino spectra from NGC 1068, NGC 3079, and NGC 4151 are softer than E⁻², indicating most neutrino power is emitted near the IceCube threshold (several hundred GeV).
• The measured neutrino fluxes are comparable to the hard X-ray luminosity of the hot corona, suggesting that proton interactions may play a key role in the energy balance of AGN coronae.