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Active Galactic Nuclei as high-energy neutrino sources

This paper reviews recent findings on the correlation between high-energy astrophysical neutrinos and Active Galactic Nuclei, highlighting confirmed associations with sources like TXS 0506+056 and NGC 1068, discussing neutrino production mechanisms, and outlining future prospects for multi-messenger astronomy.

Original authors: Filippo D'Ammando (INAF-IRA Bologna)

Published 2026-07-01

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CC BY 4.0

Original authors: Filippo D'Ammando (INAF-IRA Bologna)

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, noisy party. For a long time, astronomers could only hear the music by looking at the light (photons) bouncing off the dancers. But recently, we've learned to listen to a different kind of signal: neutrinos.

Think of neutrinos as "ghost particles." They are so tiny and have so little mass that they can walk right through walls, stars, and entire galaxies without bumping into anything. Because they don't get blocked, they can travel straight from the most dangerous, crowded rooms in the universe (like the centers of galaxies) directly to our detectors on Earth, bringing us a message that light can't carry.

This paper is a report card on our search for the "hosts" of these ghost particles. Specifically, we are looking at Active Galactic Nuclei (AGN)—super-massive black holes at the centers of galaxies that are actively eating gas and spitting out massive jets of energy.

Here is the breakdown of what the paper says, using simple analogies:

1. The Big Mystery: Who is throwing the party?

Since 2013, we've known that high-energy neutrinos exist, but we didn't know who was making them. It's like hearing a loud explosion in a dark city but not knowing which building it came from.

The Suspects: We suspected Blazars (galaxies with jets pointing directly at us, like a flashlight beam hitting your eyes) and other types of active galaxies.
The Problem: Neutrinos are hard to catch. When we catch one, we can only guess roughly where it came from. It's like catching a fly in a dark room; you know it's somewhere in the room, but you don't know exactly which corner.

2. The Two "Smoking Guns" (Confirmed Sources)

After years of searching, the paper highlights two specific galaxies that have been positively identified as neutrino factories:

TXS 0506+056 (The Flashy Blazar):
- The Analogy: Imagine a lighthouse that suddenly flashes a blinding beam of light. In 2017, a neutrino arrived at the exact same time this lighthouse (a blazar) was flashing its gamma-ray lights.
- The Discovery: This was the first time we linked a neutrino to a specific galaxy. Later, we found out this galaxy had a "neutrino party" back in 2014/2015 that we missed because we weren't looking at the right time. It seems this galaxy is a "masquerader"—it looks like a simple star-gazer (a BL Lac object) but is actually a more complex, line-heavy galaxy (an FSRQ) hiding its true nature.
- The Mechanism: The neutrinos are likely made in the high-speed jet shooting out of the black hole, where protons smash into other particles.
NGC 1068 (The Quiet Neighbor):
- The Analogy: If TXS is the lighthouse, NGC 1068 is a house with the curtains drawn. It's a "radio-quiet" galaxy. It doesn't shoot a jet at us; instead, it's a bit messy and obscured.
- The Discovery: In 2022, we found that this galaxy is actually a steady producer of neutrinos, even though it doesn't look very bright in gamma rays.
- The Mechanism: Here, the neutrinos aren't coming from a jet. They are likely being made in the "corona" (a super-hot, magnetic atmosphere) right above the black hole's dinner table (accretion disk). The black hole is so busy that the gamma rays get trapped and absorbed, but the neutrinos escape easily. It's like a factory that produces smoke (gamma rays) that gets stuck inside the building, but the workers (neutrinos) slip out the back door.

3. The "Maybe" List (Other Candidates)

The paper discusses several other galaxies that might be neutrino sources, but the evidence is shaky.

The Problem: Sometimes a neutrino arrives, and a galaxy is in the right spot, but the galaxy isn't acting "neutrino-like" (e.g., it's not flaring in gamma rays).
The Analogy: It's like hearing a crash and seeing a car nearby, but the car's engine is off. Is it the car? Maybe, or maybe it's just a coincidence.
The Candidates: Galaxies like PKS 1502+106 and PKS 0735+178 have been suggested, but the math is complicated. Sometimes the models say "yes," and sometimes they say "no," depending on how you look at the data.

4. The Search for the "Whole Crowd" (Correlations)

Instead of looking at one galaxy at a time, scientists tried to look at the whole crowd.

The Result: When they looked at thousands of gamma-ray bright galaxies (Fermi-LAT catalog) or radio-bright galaxies, they did not find a strong statistical link.
The Takeaway: This suggests that the "ghost particles" aren't coming from the most obvious, bright sources we can see. The neutrino factories might be hiding in the shadows, or they are very rare "super-achievers" rather than a common type of galaxy.

5. What's Next? (The Future)

The paper concludes that we are just getting started.

Better Detectors: We need bigger "nets" to catch more neutrinos. New telescopes are being built in the Mediterranean Sea (KM3NeT) and Lake Baikal (Baikal-GVD) to complement the one in Antarctica (IceCube).
Better Eyes: We need better cameras across the whole spectrum (from radio waves to X-rays) to catch the galaxies the moment they start "flaring."
The Goal: We want to know if these galaxies are the main source of the universe's high-energy particles, or just a small part of the story.

Summary

In short, this paper tells us that Active Galactic Nuclei are definitely making high-energy neutrinos. We have found two confirmed examples: one that shoots a jet at us (TXS 0506+056) and one that keeps its jets hidden (NGC 1068). However, we still don't know if these are the main sources of all the neutrinos in the universe, or if there are many more hiding in the dark. To solve this, we need bigger detectors and faster cameras to catch these cosmic ghosts in the act.

Technical Summary: Active Galactic Nuclei as High-Energy Neutrino Sources

Problem Statement
Since the first detection of a diffuse flux of high-energy astrophysical neutrinos by the IceCube Neutrino Observatory in 2013, identifying their specific astrophysical sources has remained a primary challenge in multi-messenger astronomy. While Active Galactic Nuclei (AGN) are established as luminous sources of high-energy $\gamma$ -rays and promising candidates for cosmic-ray accelerators, systematic searches have struggled to confirm a definitive link between the bulk of the astrophysical neutrino flux and specific AGN populations. Previous studies suggested that $\gamma$ -ray blazars alone account for only a small fraction of the total neutrino flux. Furthermore, the identification of sources is complicated by the large angular uncertainty of neutrino events and the need to distinguish between jetted (blazars) and non-jetted (radio-quiet) AGN as production sites.

Methodology
This review synthesizes observational results and theoretical models regarding the association between high-energy neutrinos and AGN. The methodology encompasses:

Case Studies: Detailed analysis of confirmed associations, specifically the blazar TXS 0506+056 and the Seyfert II galaxy NGC 1068, alongside potential candidates like PKS 1502+106, PKS 1424−418, BZB J0955+3551, and PKS 0735+178.
Multi-wavelength Correlation: Examination of temporal and spatial coincidences between neutrino alerts (from IceCube, Baikal-GVD, and KM3NeT) and electromagnetic flares across the spectrum (radio, optical, X-ray, $\gamma$ -ray).
Statistical Stacking: Review of population-based studies correlating neutrino events with catalogs of $\gamma$ -ray (Fermi-LAT) and radio-bright (RFC, MOJAVE) AGN to detect cumulative signals.
Theoretical Modeling: Evaluation of hadronic (proton-proton, $p$ - $\gamma$ ) and lepto-hadronic emission mechanisms. This includes modeling neutrino production in relativistic jets (blazars) versus dense coronae and accretion discs (Seyferts), considering constraints from Spectral Energy Distributions (SEDs) and X-ray data.

Key Contributions and Results

Confirmed Associations:
- TXS 0506+056: The detection of the 290 TeV neutrino IceCube-170922A coincident with a $\gamma$ -ray flare provided the first compelling evidence of a blazar as a neutrino source. Subsequent analysis revealed a neutrino flare in 2014–2015 without a contemporaneous $\gamma$ -ray counterpart, suggesting complex emission mechanisms or hidden neutrino production zones. The source is identified as a "masquerading" BL Lac (intrinsically an FSRQ) with a spine-layer jet structure.
- NGC 1068: IceCube identified this radio-quiet Seyfert II galaxy as a steady-state neutrino source with a significance of 4.2 $\sigma$ . The neutrino flux (1–50 TeV) significantly exceeds the observed $\gamma$ -ray flux, implying that $\gamma$ -rays are absorbed within the source (likely by the dense corona) while neutrinos escape. This supports a production mechanism in the magnetized corona surrounding the supermassive black hole via photo-meson interactions.
Potential Associations: Several other blazars (e.g., PKS 1502+106, PKS 0735+178) have shown temporal/spatial coincidences with neutrino events. However, these associations often lack the statistical significance of TXS 0506+056 or NGC 1068, with some models suggesting the neutrino production may be subdominant or decoupled from the observed electromagnetic flares.
Population Correlations: Stacking analyses of Fermi-LAT blazars and radio-bright AGN (RFC, MOJAVE) have generally failed to find statistically significant correlations with the diffuse neutrino flux (p-values often > 0.05). While some studies claimed correlations with radio flares, these results are contested by more rigorous analyses that do not artificially inflate error regions.
Production Mechanisms:
- Jetted AGN (Blazars): Neutrinos are likely produced via $p$ - $\gamma$ interactions in the relativistic jet, potentially involving external photon fields (BLR, torus) or internal synchrotron photons.
- Non-Jetted AGN (Seyferts): Neutrinos are produced in the dense, hot corona via $p$ - $\gamma$ interactions. The corona absorbs the resulting $\gamma$ -rays, explaining the lack of high-energy $\gamma$ -ray counterparts.

Significance and Future Prospects
The paper asserts that the identification of both TXS 0506+056 and NGC 1068 as neutrino sources demonstrates that different classes of AGN (jetted and non-jetted) act as cosmic neutrino factories, though they likely contribute to different energy ranges of the diffuse flux (blazars at $E > 1$ PeV, Seyferts at 1–100 TeV).

The review highlights that while individual sources contribute minimally to the total diffuse flux (estimated at a few percent), non-jetted AGN could theoretically account for up to 95% of the flux in the 1–100 TeV range. The paper emphasizes that the field is entering a critical phase where:

Next-Generation Facilities: Upcoming detectors like IceCube-Gen2, KM3NeT, and Baikal-GVD will improve angular resolution and sensitivity, particularly in the Southern Hemisphere and at ultra-high energies (up to EeV).
Multi-Messenger Synergy: Future progress depends on coordinated, real-time follow-up across the electromagnetic spectrum. New facilities such as CTAO (VHE $\gamma$ -rays), COSI and AMEGO-X (MeV gap), IXPE (X-ray polarization), and the Vera C. Rubin Observatory (optical) are essential for characterizing source physics and breaking degeneracies in emission models.
Open Questions: Key challenges remain in understanding the exact production sites within AGN, the role of Doppler boosting, and the nature of the ultra-high-energy neutrino detected by KM3NeT (~220 PeV), which may indicate a new class of accelerators.

In conclusion, the paper posits that a unified approach combining improved neutrino detection capabilities with comprehensive multi-wavelength monitoring is necessary to fully resolve the origin of high-energy astrophysical neutrinos and their connection to AGN.