Neutrino-Emitting AGN

• Neutrino-emitting AGN are extragalactic sources where ultra-high-energy particles interact via photo-hadronic and hadronuclear processes to generate neutrinos.
• They feature complex environments—ranging from jet-dominated regions to dense Seyfert cores—with efficient particle acceleration and localized neutrino production near supermassive black holes.
• Observations (e.g., from IceCube) and multimessenger studies link these AGN to key spectral features and energy budgets, underscoring their role in cosmic-ray acceleration and gamma-ray attenuation.

Neutrino-emitting active galactic nuclei (AGN) are a class of extragalactic sources in which relativistic particles—primarily protons and heavier ions—are accelerated to ultra-high energies and subsequently interact with dense photon fields or ambient matter, generating high-energy neutrinos through hadronic processes. These environments comprise luminous accretion disks and hot coronae around supermassive black holes, powerful jets, broad-line region clouds, and, in some scenarios, intense radiation-driven winds. Observational and theoretical studies demonstrate that such AGN, including both jet-dominated ("jet-loud") and non-jetted ("jet-quiet") systems, play a significant role in the multi-messenger astrophysical landscape, linking neutrino emission, cosmic-ray acceleration, and electromagnetic counterparts across the spectrum.

1. Physical Mechanisms of Neutrino Production

High-energy neutrino production in AGN proceeds via two principal hadronic channels: photo-hadronic ($p\gamma$) and hadronuclear ($pp$) interactions. In $p\gamma$ scenarios, relativistic protons interact with ambient photon fields (typically from the accretion disk, corona, or broadline region), producing charged and neutral pions through reactions such as

$$p + \gamma \rightarrow \Delta^+ \rightarrow \begin{cases} p + \pi^0, \ n + \pi^+ \end{cases}$$

with $\pi^+ \rightarrow \mu^+ + \nu_\mu$, $\mu^+ \rightarrow e^+ + \nu_e + \bar\nu_\mu$, and $\pi^0\rightarrow 2\gamma$. In $pp$ interactions, the dominant chain is

$p + p \rightarrow p + n + \pi^\pm/\pi^0\,,$

with a similar decay sequence for $\pi^\pm$. The resultant high-energy neutrino spectrum arises from the convolution of the primary proton spectrum, the energy-dependent cross-sections ($\sigma_{p\gamma}(E)$, $\sigma_{pp}(E)$), the photon and/or nucleon density, and the target photon spectra (beamed, disk, BLR, IR/dust, coronal X-ray, depending on environment) (Arteaga-Velazquez et al., 2013, Murase et al., 2014, Murase, 2015, Murase et al., 2022). The efficiency of pion production (often parametrized as $f_{p\gamma}$ or $f_{pp}$) is set by the optical depth through regions of large photon or matter density.

The differential neutrino luminosity per solid angle can be written in the photo-hadronic case as

$$\frac{dL_\nu(E_\nu, \theta, i)}{d\Omega} dE_\nu = \Sigma_{\mathrm H}(\theta) \int_{E_{\gamma i}}^{E_{\gamma f}} d E_\gamma \, Y^{\gamma P\rightarrow \nu}(E_\gamma, E_\nu)\, \sigma_{\gamma P}(E_\gamma) \frac{dL_\gamma(E_\gamma,\theta,i)}{d\Omega}$$

where $\Sigma_{\mathrm H}$ is the column density of target protons, and $Y^{\gamma P\rightarrow \nu}$ is the neutrino yield function (Arteaga-Velazquez et al., 2013). In hadronuclear environments, the proton energy loss rate is $t_{pp}^{-1}=n_N \kappa_{pp} \sigma_{pp} c$ (Murase et al., 2022).

In the most general models, both channels may be active, with their relative contributions determined by local densities, photon fields, and magnetic turbulence (Murase, 2022, Huang et al., 20 Jun 2024).

2. AGN Environments and Their Role in Neutrino Production

Neutrino emission in AGN is fundamentally linked to the diverse environments present across AGN types:

• Jet-loud AGN (blazars, radio galaxies): In inner jets, protons are accelerated and interact with internal synchrotron/Compton photons and external BLR and IR dust photon fields (Murase et al., 2014). The presence of strong external photon fields, most notably Ly$\alpha$ from the BLR and IR dust emission, induces energy-dependent cutoffs and spectral hardening in the neutrino spectrum around PeV-EeV; the typical photopion efficiency within the BLR can reach $f_{p\gamma} \sim 0.01 - 0.1$ (Murase, 2015).
• Jet-quiet AGN (Seyfert galaxies, non-blazar AGN): Here, neutrino emission is attributed to cosmic-ray interactions in the corona, accretion disk, or with dense outflows and BLR clouds (Kheirandish et al., 2021, Murase, 2022, Huang et al., 20 Jun 2024, Romero et al., 25 Aug 2025). Magnetized coronae above the disk are sites for efficient particle acceleration via magnetic reconnection or turbulent processes. Photo-meson production with coronal X-rays dominates at smaller radii, while hadronuclear processes can become efficient in dense winds or outflow-cloud interaction regions.
• Outflow and wind scenarios: During super-Eddington accretion episodes, radiation-driven winds collide with BLR clouds, forming bowshocks where protons are efficiently accelerated via diffusive shock acceleration and then undergo $pp$ collisions with cloud material, producing neutrinos and suppressed gamma-ray emission due to the extreme photon density (Huang et al., 20 Jun 2024, Romero et al., 25 Aug 2025).

Notably, stellar-mass black holes embedded in AGN disks may also launch jets via the Blandford–Znajek mechanism, providing localized non-thermal regions capable of additional $p\gamma$ or $pp$ processes (Tagawa et al., 2023).

3. Observational Evidence and Constraints

Recent observations, particularly by IceCube, provide crucial constraints and identification of neutrino-emitting AGN. Notable findings include:

• NGC 1068: Identified as a significant neutrino "hot spot" with ∼79 events and a 4.2σ excess in the 1–10 TeV energy range (Halzen, 2023, Abbasi et al., 2021). The source is characterized by bright X-ray emission from a hot, optically thick corona and exhibits heavy gamma-ray obscuration; neutrinos are produced within $10–100$ Schwarzschild radii, and the gamma rays associated with hadronic processes are efficiently absorbed and reprocessed within the core (Murase, 2022, Huang et al., 20 Jun 2024).
• TXS 0506+056: Previously detected in directional and temporal coincidence with neutrino and gamma-ray flares (Halzen, 2023, Zathul et al., 21 Nov 2024); although often associated with blazar jet scenarios, analyses support an alternative interpretation in which neutrino emission also stems from an obscured core, aligned with the same physical process as inferred in NGC 1068 (Zathul et al., 21 Nov 2024).
• Population constraints: Stacking analyses and population studies using Fermi-LAT and IceCube data reveal that classical gamma-ray-bright blazars contribute no more than $5–15\%$ of the diffuse astrophysical neutrino flux; non-blazar AGN, less luminous and without strong gamma-ray signatures, are the most plausible candidates for the majority of the observed flux (Hooper et al., 2018, Hori et al., 9 Jul 2025).
• Energy budgets: To match the observed diffuse PeV neutrino flux, for example, LLCD-based models require $\sim 5–7\%$ of the AGN bolometric luminosity to be converted into relativistic proton kinetic energy (Dey et al., 2022). For blazar-dominated scenarios, required cosmic ray energization rates may exceed the local UHECR energy budget by up to two orders of magnitude unless the proton spectral index is steepened or the maximum energy limited (Murase et al., 2014).

4. Spectral Features, Multiplicity, and Multimessenger Connections

The spectral features of the neutrino flux reflect the interplay of acceleration, interaction rates, and environmental photon densities:

• Spectral cutoffs and hardening: The dominance of BLR Ly$\alpha$ photons as targets yields an intense neutrino flux with a cutoff at PeV energies, while IR dust photons result in hard spectra above PeV, detectable by next-generation observatories at EeV energies (Murase et al., 2014).
• Multimessenger correlations: The photon–neutrino relation $$\epsilon_\gamma L_\gamma^{(\text{gen})} \approx (4/3 K) [\epsilon_\nu L_\nu]_{\epsilon_\nu = \epsilon_\gamma / 2}$$ (with $K=1$ for $p\gamma$, $K=2$ for $pp$) sets a generic expectation for comparable gamma-ray and neutrino fluxes, modulo in-source gamma-ray attenuation (Murase, 2022).
• Radio–neutrino correlation: If radio emission arises from synchrotron of secondary electrons from charged pion decay in jets, a direct radio–neutrino connection emerges, shown to underlie observed angular correlations between radio-bright AGN and IceCube events (Neronov et al., 2020).
• Gamma-ray—neutrino disconnect: In heavily obscured AGN cores and wind–cloud scenarios, dense photon fields lead to high $\gamma\gamma$ opacity, with neutrinos escaping but gamma rays absorbed and reprocessed below the GeV regime (Murase, 2022, Huang et al., 20 Jun 2024, Halzen, 2023).

5. Alternative and Hybrid Mechanisms

Beyond hadronic interactions, alternative models exist:

• Purely leptonic neutrino production: High-energy electrons in AGN coronae can generate TeV neutrinos through $\gamma\gamma$ interactions—in particular, very high-energy photons (TeV-scale) scatter with coronal X-rays and produce $\mu^+\mu^-$ pairs, whose subsequent decay yields neutrinos, all without proton acceleration (Hooper et al., 2023). This mechanism requires both a source of TeV photons and a dense keV-dominated radiation field, as observed in NGC 1068.
• Super-accreting AGN scenarios: During super-Eddington accretion, radiation-driven winds overwhelm BLR clouds, forming shocks that accelerate particles. The resulting pp neutrino flux is set by the coverage and filling factor of the BLR and may lead to "orphan" neutrino sources—AGN without gamma-ray counterparts but detectable in neutrinos when the BLR filling fraction is high (Romero et al., 25 Aug 2025). The timescale of emission is tied to the duration of the super-accretion phase and the number of active cloud–wind interactions.

6. Open Questions and Future Prospects

Recent results sharpen, but do not close, several debates:

• Population dominance: Analyses using thirteen years of IceCube data find no cumulative neutrino signal from Fermi-LAT–detected AGN populations, strongly constraining scenarios in which simple correlations between gamma-ray and neutrino flux (linear or quadratic scaling) would account for the observed diffuse neutrino intensity (Hori et al., 9 Jul 2025). This suggests a need to reconsider either the physical scaling relations or invoke rare subclasses of AGN or states of activity.
• Source localization and event multiplicity: The identification of individual (steady or flaring) AGN as neutrino sources remains rare. NGC 1068, PKS 1424+240, and TXS 0506+056 have been singled out, but statistical association across source catalogs remains below discovery threshold, with post-trial significances ranging from 2–4σ (Halzen, 2023, Abbasi et al., 2021, Hori et al., 9 Jul 2025).
• Environment-driven phenomenology: The contrast between blazar and Seyfert-like emission (jet-dominated versus core-dominated) in neutrino output, gamma-ray attenuation, and X-ray activity underscores the need for environmental diagnostics, especially at sub-parsec (tens of gravitational radii) scales. The next generation of observatories with improved angular and energy resolution (e.g. IceCube-Gen2, KM3NeT, AMEGO-X) will refine stacking analyses and probe for both resolved and "hidden" neutrino sources in the local universe (Kheirandish et al., 2021, Murase, 2022).
• Non-standard signatures: Cosmic-ray interactions in the central regions of neutrino-emitting AGN may produce boosted dark matter fluxes observable in direct detection experiments, offering a novel probe of the dark sector that is otherwise inaccessible to standard searches (Gustafson et al., 28 Aug 2025).

7. Conclusion

Neutrino-emitting active galactic nuclei form a heterogeneous class, encompassing blazars, radio galaxies, Seyfert cores, and transient super-accretors. Their paper has illuminated the multi-component nature of cosmic-ray acceleration and high-energy neutrino production, the critical importance of environment in mediating observed signals, and the necessity of multi-messenger and multi-wavelength observations for disentangling the underlying physical processes. The emerging picture points to obscured, dense cores within $10–100$ Schwarzschild radii as canonical neutrino emission sites, with gamma rays largely absorbed and only secondary electromagnetic emission escaping. Population studies and theoretical models suggest that a significant fraction of the observed diffuse astrophysical neutrino flux originates from non-blazar, often gamma-ray-dark, AGN and from rare or highly energetic periods—opening ongoing avenues for discovery as observational capabilities and multi-messenger methodologies continue to evolve.