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Revisiting Disk Winds in Active Galactic Nuclei as an Origin of Cosmic Gamma-ray and Neutrino Backgrounds

By constructing a lepto-hadronic wind model calibrated with multi-wavelength data and XRISM constraints, this study demonstrates that active galactic nucleus disk winds contribute at most 5% to the cosmic gamma-ray background and 10% to the cosmic neutrino background, indicating they are unlikely to be the dominant origin of these backgrounds.

Original authors: Nobuyuki Sakai, Yoshiyuki Inoue, Ellis R. Owen

Published 2026-06-17

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CC BY 4.0

Original authors: Nobuyuki Sakai, Yoshiyuki Inoue, Ellis R. Owen

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

The Big Mystery: Where Do the Universe's "Ghost Particles" Come From?

Imagine the universe is filled with a constant, invisible rain of high-energy particles. Some are gamma rays (super-powerful light), and others are neutrinos (ghost-like particles that can pass through planets without hitting anything). Scientists have been trying to figure out exactly where this "cosmic rain" comes from.

One popular theory suggests that Active Galactic Nuclei (AGNs)—the super-bright, hungry hearts of galaxies powered by giant black holes—are the source. Specifically, scientists thought that winds blowing out from the swirling disks of gas around these black holes might be the "shovels" digging up these particles.

The New Study: Checking the "Wind" Theory

The authors of this paper, Nobuyuki Sakai and his team, decided to test this "wind theory" more carefully than before. They built a detailed computer model to see if these galactic winds could actually produce enough gamma rays and neutrinos to explain the cosmic background we see.

Think of their model like a wind tunnel experiment for the universe. They simulated what happens when a super-fast wind from a black hole crashes into the surrounding gas of its galaxy.

How the "Crash" Works:

The Wind: A black hole blows a massive wind of gas and energy.
The Collision: This wind slams into the slower gas sitting around the galaxy (like a firehose hitting a wall of fog).
The Shocks: This crash creates two "shockwaves" (like sonic booms).
- A Forward Shock pushes into the fog.
- A Reverse Shock bounces back into the wind.
The Acceleration: These shockwaves act like giant particle accelerators. They smash particles (protons and electrons) to near the speed of light, turning them into cosmic rays.
The Result: These super-fast particles then crash into each other or light waves, creating the gamma rays and neutrinos we are trying to detect.

The Calibration: Testing on "Neighborhood" Galaxies

Before looking at the whole universe, the team needed to make sure their model worked. They picked five nearby galaxies (Seyfert galaxies) that we know have these winds and have already been spotted by the Fermi-LAT telescope (which sees gamma rays).

They adjusted their model's "knobs" (like wind speed, gas density, and magnetic field strength) until the model perfectly matched the real data from these five galaxies. They tested two main scenarios:

Scenario A (The Light Show): The gamma rays are mostly made by electrons bouncing off light waves (like a disco ball).
Scenario B (The Crash): The gamma rays are mostly made by protons smashing into each other (like a car crash).

Both scenarios worked well to explain the nearby galaxies.

The Big Finding: The Wind is a Minor Player

Once the model was calibrated, they used it to calculate the total contribution of these winds to the entire universe's background radiation.

The Verdict:
The winds are real, and they do produce high-energy particles, but they are not the main source of the cosmic background.

For Gamma Rays: The winds contribute less than 5% of the total gamma-ray background above 10 GeV.
For Neutrinos: The winds contribute less than 10% of the total neutrino background around 100 TeV.

The Analogy:
Imagine the cosmic background is a giant soup. Scientists have been trying to figure out what the main ingredient is. This paper says, "We checked the 'Galactic Wind' ingredient, and while it adds a little flavor, it's definitely not the main broth." The soup is likely made of other things (like blazars or star-forming galaxies) that we haven't fully pinned down yet.

Why Was the Previous Theory Overestimating?

The authors found that earlier studies predicted much higher contributions because they assumed the winds were hitting extremely dense gas clouds (like a firehose hitting a brick wall).

However, the authors incorporated new data from the XRISM satellite (a recent X-ray observatory). This new data suggests that while the winds are powerful, the gas they hit is usually much less dense (more like a firehose hitting a thick fog). When you hit a less dense target, you don't create as many high-energy particles. This adjustment lowered their predictions significantly.

What's Next? The Future Hunt

Even though the winds aren't the main source, they are still interesting. The paper suggests that future telescopes could give us a "smoking gun" to prove this theory once and for all.

New Tools: Telescopes like CTAO (which sees very high-energy gamma rays) and TRIDENT (a future neutrino detector) will be able to look at these specific Seyfert galaxies in much greater detail.
The Goal: If these new telescopes detect a specific mix of gamma rays and neutrinos from these galaxies that matches the "wind crash" model, it will confirm that the physics in the paper is correct.

Summary

The paper says: "We built a better model of how black hole winds work, calibrated it with real nearby galaxies, and found that while these winds are energetic, they are only a small side-dish in the universe's high-energy menu. They aren't the main course."

Technical Summary: Revisiting Disk Winds in Active Galactic Nuclei as an Origin of Cosmic Gamma-ray and Neutrino Backgrounds

Problem Statement
The origins of the cosmic neutrino background (CNB) and the cosmic gamma-ray background (CGB) remain uncertain. While various sources such as blazars, star-forming galaxies, and AGN coronae have been proposed, their contributions are either limited by stacking analyses or constrained by multi-messenger flux limits (e.g., blazars contributing $\lesssim 30\%$ to the TeV–PeV CNB, and star-forming galaxies potentially overshooting the CGB if they account for the full CNB). AGN disk winds, launched from accretion disks around supermassive black holes (SMBHs), have been proposed as alternative contributors. However, previous estimates of their contribution to the CNB and CGB span a wide range (from $\sim 10\%$ to nearly $100\%$ ), primarily due to poorly constrained model parameters such as wind kinetic efficiency, ambient gas densities, and wind ages.

Methodology
The authors construct a self-consistent lepto-hadronic wind model to revisit the AGN disk-wind scenario. The methodology involves:

Dynamical Evolution: Modeling the wind as a spherically averaged system where a steady-state cosmic ray (CR) distribution is accelerated at forward shocks (driven into the interstellar medium, ISM) and reverse shocks (driven into the wind). The model incorporates recent constraints from the X-Ray Imaging and Spectroscopy Mission (XRISM) regarding wind energetics.
Particle Physics: Calculating the steady-state CR spectrum by solving the transport equation, accounting for diffusive shock acceleration (DSA), adiabatic losses, advection, and radiative losses (synchrotron, inverse Compton, $pp$, $p\gamma$ , and Bethe-Heitler pair production).
Calibration: Calibrating the model parameters against radio and GeV gamma-ray data from five nearby Fermi-LAT-detected Seyfert galaxies (Circinus, NGC 4945, NGC 4151, NGC 1068, and GRS 1734–292). Two limiting scenarios are explored to address parameter degeneracies:
- Model A: GeV emission is dominated by external Compton (EC) scattering in the shocked wind (SW).
- Model B: GeV emission is dominated by hadronic ($pp$) interactions in the shocked ambient medium (SAM).
Population Synthesis: Applying the calibrated parameters to an AGN population synthesis model (integrating over redshift, luminosity, and wind age) to estimate the cumulative contribution of disk winds to the diffuse CGB and CNB.

Key Contributions

Calibrated Lepto-Hadronic Framework: The paper provides a unified model that simultaneously reproduces observed radio and GeV fluxes for a sample of Seyfert galaxies, deriving physically plausible wind boost factors ( $b_w \sim \text{few to } 130$ ), magnetic fields, and shock radii.
Parameter Constraints: By fitting to specific nearby sources, the authors constrain the wind boost factor and ambient gas density, moving beyond the broad assumptions of previous studies which often assumed unrealistically high gas densities ( $\sim 10^3 \text{ cm}^{-3}$ ) or low kinetic efficiencies.
Robustness Check: The study explicitly tests the sensitivity of the results to key parameters, including the wind boost factor ( $b_w$ ), wind age ( $t_w$ ), ISM density ( $n_0$ ), and the ISM density profile index ( $\beta$ ).

Results

Contribution to Backgrounds: When applied to the AGN population, the calibrated models indicate that disk winds contribute at most $\lesssim 5\%$ of the CGB above 10 GeV and $\lesssim 10\%$ of the CNB around 100 TeV.
Dominance: The results suggest that AGN disk winds are unlikely to be the dominant source of either the CGB or the CNB. The contribution remains subdominant even under optimistic assumptions, such as assuming all AGNs host winds ( $f_w=1$ ) and using a maximum opening solid angle ( $\Omega_w = 4\pi$ sr).
Model Comparison: Both Model A (EC-dominated) and Model B (hadron-dominated) yield similar background spectra because hadronic emission constitutes a substantial fraction ( $\sim 40-50\%$ ) of the disk-wind CGB in both scenarios.
Comparison with Previous Works: The authors' predicted background intensities are significantly lower (by factors of $\sim 5$ to $100$) than those predicted by previous studies (e.g., Wang & Loeb 2016a; Lamastra et al. 2017; Liu et al. 2018). This discrepancy is attributed to the previous works' assumptions of much denser circumnuclear gas, the lack of proton cooling in some models, and the assumption that all AGNs host winds.
Future Observability: The paper identifies nearby Seyfert galaxies hosting ultrafast outflows (UFOs), such as Circinus, NGC 4945, and NGC 1068, as promising targets for future TeV gamma-ray (CTAO, LHAASO, SWGO) and TeV–PeV neutrino (IceCube-Gen2, TRIDENT) observations. These observations could provide firm tests of the disk-wind scenario.

Significance
The paper concludes that while AGN disk winds can efficiently accelerate cosmic rays and contribute non-negligible fractions to the high-energy backgrounds, they are not the primary origin of the diffuse CGB or CNB. The study emphasizes that the contribution is highly dependent on wind incidence, ambient density, and energetics. By calibrating against specific nearby sources and incorporating recent XRISM constraints, the authors provide a more realistic upper bound on the disk-wind contribution. The work highlights the necessity of future multi-messenger observations (combining X-ray, radio, GeV–TeV gamma-ray, and neutrino data) to further constrain wind parameters and definitively test the efficiency of CR acceleration in AGN cores.