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Constraining the contribution of Seyfert galaxies to the diffuse neutrino flux in light of point source observations

By modeling neutrino emission from Seyfert galaxies based on stochastic proton acceleration in magnetized coronae and validating against IceCube and Fermi-LAT data, the study finds that while these galaxies can explain a significant fraction of the diffuse neutrino flux below 10 TeV, the most efficient emitters like NGC 1068 are exceptional outliers whose parameters cannot be universally applied to the entire population without violating current TeV upper limits.

Original authors: Lena Saurenhaus, Francesca Capel, Foteini Oikonomou, Johannes Buchner

Published 2026-03-13

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CC BY 4.0

Original authors: Lena Saurenhaus, Francesca Capel, Foteini Oikonomou, Johannes Buchner

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 is a giant, noisy concert hall. For years, scientists have been trying to figure out where the loudest, most energetic "notes" (called neutrinos) are coming from. These are ghostly particles that zip through space and even through the Earth without stopping.

Recently, the IceCube observatory (a giant detector buried in the Antarctic ice) found a strong signal coming from a specific "singer" in the Northern sky: a galaxy called NGC 1068. This galaxy is a "Seyfert galaxy," which is essentially a galaxy with a supermassive black hole at its center that is actively eating matter and glowing brightly in X-rays.

This paper is like a detective story where the authors try to answer two big questions:

How does NGC 1068 make so many neutrinos?
If NGC 1068 is doing it, are all Seyfert galaxies doing it too? If so, shouldn't the whole sky be flooded with neutrinos?

Here is the breakdown of their investigation using simple analogies.

1. The "Kitchen" Inside the Galaxy

The authors propose a model for how these galaxies work. Imagine the center of the galaxy (around the black hole) has a super-hot, turbulent "kitchen" called a corona.

The Ingredients: Inside this kitchen, there are protons (particles of matter) and X-ray light.
The Chef: The "chef" is a chaotic, magnetic storm (turbulence) that acts like a cosmic particle accelerator. It slams the protons around, speeding them up to incredible energies.
The Cooking: When these super-fast protons crash into other particles or light, they explode into a shower of new particles, including neutrinos and gamma rays (high-energy light).

2. The "Hidden" Chef (Why we don't see the light)

Here is the tricky part. If you cook a meal that produces neutrinos, physics says it should also produce a lot of bright gamma rays. But when we look at NGC 1068 with our telescopes, we see the neutrinos, but we don't see the gamma rays.

The authors explain this with a metaphor: The Foggy Room.
Imagine the kitchen is a small, dense room filled with thick fog (X-ray photons). When the gamma rays try to leave the room, they crash into the fog and get absorbed or scattered before they can escape. However, neutrinos are like ghosts; they walk right through the fog and the walls without hitting anything.

The Clue: Because the gamma rays are blocked, the authors realized the "kitchen" (the corona) must be very small and dense. They calculated it must be smaller than 5 times the size of the black hole's event horizon. If it were bigger, the gamma rays would escape, and we would have seen them.

3. The "Crowded Concert Hall" Problem

Now, the authors ask: "If NGC 1068 is such a great neutrino chef, what about the other 100,000+ Seyfert galaxies in the universe?"

They ran a simulation assuming every single Seyfert galaxy was exactly like NGC 1068 (same size kitchen, same chaotic storm, same cooking efficiency).

The Result: If every galaxy cooked at that level, the "concert hall" (the universe) would be so loud with neutrinos that it would drown out everything else. The total amount of neutrinos predicted would be 3.8 times higher than what IceCube actually sees.
The Verdict: This is a "no-go" zone. It's impossible that all these galaxies are as efficient as NGC 1068.

4. The "Star Performer" Conclusion

So, what does this mean?

NGC 1068 is a Superstar: It is an outlier. It is an exceptionally efficient neutrino factory, likely because its "kitchen" is perfectly set up for chaos and acceleration.
The Rest are Background Noise: Most other Seyfert galaxies are much less efficient. They might have smaller storms, less turbulence, or just aren't cooking as hard. They contribute to the background hum of neutrinos, but they aren't the main event.

The Takeaway

The paper concludes that while Seyfert galaxies are important contributors to the universe's neutrino background (especially at lower energies), the specific galaxy NGC 1068 is a unique, extreme case.

In simple terms:
If the universe's neutrino flux is a soup, NGC 1068 is a giant, concentrated spoonful of flavor. The authors proved that if you added a spoonful of that flavor to every bowl in the world, the soup would be inedible. Therefore, most bowls must have very little flavor, and only a few special bowls (like NGC 1068) have the concentrated stuff.

This discovery helps scientists understand that the "hidden" neutrino sources we see are rare, special objects, not the standard rule for all active galaxies. It also tells us that to find more of these, we need to look for galaxies with very specific, extreme conditions, and we need better telescopes (like the upcoming IceCube-Gen2) to spot the fainter ones.

1. Problem Statement

The origin of the diffuse astrophysical neutrino flux observed by the IceCube Neutrino Observatory remains a major open question in high-energy astrophysics. While IceCube has recently identified a significant excess of neutrinos from the nearby Seyfert galaxy NGC 1068 (and hints from NGC 4151 and CGCG 420-015), it is unclear whether these specific sources are representative of the broader population of Seyfert galaxies.

The core problem addressed is: Can the entire population of Seyfert galaxies, assuming they share the same physical properties as NGC 1068, account for the observed diffuse neutrino flux without violating current observational limits (specifically at TeV energies and MeV $\gamma$ -ray limits)?

2. Methodology

The authors employ a multi-messenger approach combining neutrino physics, $\gamma$ -ray constraints, and population synthesis.

A. Neutrino Emission Model (Single Source)

Physical Scenario: Based on the disk-corona model, the authors assume protons are accelerated via stochastic acceleration (second-order Fermi acceleration) in magnetized turbulence within the AGN corona.
Interactions: Accelerated protons interact with ambient gas ($pp$) and coronal X-rays ( $p\gamma$ ) to produce neutrinos and $\gamma$ -rays.
$\gamma$ -ray Attenuation: High-energy $\gamma$ -rays produced in the corona are attenuated via $\gamma\gamma$ interactions with X-rays, initiating electromagnetic cascades that escape at MeV energies.
Simulation: The authors use the multi-messenger simulation code AM3 to calculate neutrino and cascaded $\gamma$ -ray spectra.
Key Parameters:
- $L_X$ : Intrinsic 2–10 keV X-ray luminosity.
- $r_c$ : Dimensionless coronal radius ( $R_c = r_c R_S$ , where $R_S$ is the Schwarzschild radius).
- $\eta$ : Inverse turbulence strength (determines acceleration efficiency).
- $P_{CR}/P_{th}$ : Ratio of cosmic-ray pressure to thermal gas pressure (normalization of the proton spectrum).

B. Fitting to NGC 1068

The model is fitted to the public 10-year IceCube dataset (track-like events) for NGC 1068 using the unbinned likelihood software SkyLLH.
The fit determines the best-fit values for $\eta$ and the number of signal events ( $n_s$ ), which constrains $P_{CR}/P_{th}$ .
The model's $\gamma$ -ray predictions are compared against Fermi-LAT observations to constrain the coronal radius ( $r_c$ ).

C. Population Synthesis

The authors extrapolate the model to the full Seyfert population using three different X-ray Luminosity Functions (XLFs):
1. U14 (Ueda et al.): Parametric broken power law.
2. B15 (const. slope) and B15 (const. value) (Buchner et al.): Non-parametric approaches.
Correction for Bright Sources: To accurately model the local universe, sources with flux $F_X \geq 10^{-11}$ erg cm $^{-2}$ s $^{-1}$ in the simulated populations are replaced by actual sources from the BAT AGN Spectroscopic Survey (BASS) catalog.
Diffuse Flux Calculation: The neutrino flux from all sources is summed, accounting for cosmological redshift and adiabatic expansion, to generate the predicted diffuse flux.

D. Constraints

The predicted diffuse flux is compared against IceCube measurements (ESTES starting tracks and cascade events).
The authors perform a likelihood analysis to exclude regions of the parameter space ( $P_{CR}/P_{th}$ vs. $\eta$ ) where the cumulative flux exceeds IceCube upper limits.

3. Key Contributions

First Detailed Multi-Messenger Constraints: The paper provides the first rigorous constraints on a physically motivated neutrino emission model for Seyfert galaxies, informed simultaneously by the point-source detection of NGC 1068 (neutrinos), Fermi-LAT data ( $\gamma$ -rays), and the diffuse flux limits.
Quantification of NGC 1068's Uniqueness: The study demonstrates that NGC 1068 is an outlier. Its physical parameters (specifically high CR pressure and moderate turbulence) are required to fit its specific neutrino signal but, if applied to the whole population, would overproduce the diffuse flux.
Refinement of X-ray Luminosity Functions: The authors validate that the B15 constant-slope XLF provides a more realistic description of the local bright Seyfert population than the B15 constant-value prior, which overestimates the number of high-luminosity sources.
Coronal Radius Constraint: By comparing cascaded $\gamma$ -ray predictions to Fermi-LAT data, the authors constrain the coronal radius of NGC 1068 to $R_c \lesssim 5 R_S$ .

4. Key Results

Best-Fit Parameters for NGC 1068:
- Inverse turbulence strength: $\hat{\eta} \approx 56$ .
- Pressure ratio: $P_{CR}/P_{th} = 0.5$ (the maximum physically allowed value before the corona becomes unstable).
- Coronal radius: $r_c \leq 5$ .
- These parameters imply NGC 1068 hosts an exceptionally efficient hadronic accelerator.
Diffuse Flux Exclusion:
- If all Seyfert galaxies shared NGC 1068's parameters, the cumulative diffuse neutrino flux would exceed current IceCube upper limits at TeV energies by 3.8 $\sigma$ (specifically violating the ESTES analysis limit at $\sim$ 2 TeV).
- Scenarios with high CR pressure ( $P_{CR}/P_{th} > 0.2$ ) combined with high turbulence ( $\eta < 40$ ) are ruled out for the general population.
Population Requirements:
- To explain the diffuse flux below $\sim$ 10 TeV without violating limits, the majority of Seyfert galaxies must have low CR pressure ( $P_{CR}/P_{th} \lesssim 0.05$ ) and high turbulence ( $\eta \lesssim 10$ ).
- Under this scenario, NGC 1068 (and potentially NGC 4151 and CGCG 420-015) represents a rare, highly efficient subset of the population, contributing disproportionately to the point-source signal but not the bulk of the diffuse flux.
XLF Comparison:
- The choice of XLF has a minor impact on the total diffuse flux (differing by $<2\times$ ) because the flux is dominated by the total X-ray energy density. However, the B15 constant-value prior predicts too many bright sources to be consistent with the BASS catalog.

5. Significance

Resolution of the "Hidden Source" Tension: The results support the hypothesis that Seyfert galaxies are "hidden" neutrino sources (where $\gamma$ -rays are absorbed), but clarify that they cannot be the sole source of the diffuse flux if they are all as efficient as NGC 1068.
Implications for Future Observations: The paper suggests that future neutrino telescopes (IceCube-Gen2, KM3NeT) and MeV $\gamma$ -ray missions (e.g., AMEGO-X, COSI) are crucial. KM3NeT, in particular, is highlighted for its sensitivity to Southern Hemisphere Seyferts (e.g., Circinus, Centaurus A), which are X-ray bright but currently unobserved in neutrinos.
Theoretical Shift: The findings challenge simple scaling laws (where neutrino luminosity scales linearly with X-ray luminosity). Instead, they suggest a complex distribution of acceleration efficiencies and turbulence levels across the AGN population, where only a small fraction ( $\lesssim 25\%$ ) are "neutrino loud."

In conclusion, the paper establishes that while Seyfert galaxies can account for a significant fraction of the diffuse neutrino flux below 10 TeV, the specific conditions required to explain NGC 1068 are too extreme to apply universally. NGC 1068 is likely a unique, highly efficient neutrino emitter rather than a typical member of its class.