Seyfert Galaxies as Neutrino Sources: An Outflow$-$Cloud Interaction Perspective

TL;DR

This study posits that Seyfert galaxies produce high-energy neutrinos chiefly through proton–proton interactions arising from AGN-driven outflow–cloud bow shocks in the corona region. The authors develop and apply an outflow–cloud interaction model, showing that protons accelerated at bow shocks interact with ambient protons to yield neutrinos, with a subdominant contribution from pγ processes at the highest energies. By fitting five neutrino-associated Seyferts and evaluating Fermi-LAT gamma-ray constraints, the work demonstrates that the model can reproduce observed TeV neutrino fluxes and remains compatible with gamma-ray limits. Population-level calculations using X-ray luminosity functions indicate Seyfert galaxies can significantly contribute to the diffuse neutrino background in the 10^4–10^5 GeV range, while constraining cloud locations to avoid overproduction of gamma rays; future observations by IceCube-Gen2, KM3NeT, and MeV–GeV gamma-ray missions will further test this scenario.

Abstract

Following the identification of the first confirmed individual neutrino source, Seyfert galaxies have emerged as the most prominent class of high-energy neutrino emitters. In this work, we perform a detailed investigation of the outflow--cloud interaction scenario for neutrino production in Seyfert nuclei. In this framework, fast AGN-driven winds collide with clumpy gas clouds in the nuclear region, forming bow shocks that efficiently accelerate cosmic-ray protons. The accelerated protons subsequently interact with cold protons from the outflows via inelastic proton--proton ($pp$) collisions, producing high-energy neutrinos, while the photomeson ($pγ$) process with disk photons may provide a subdominant contribution at the highest energies. Applying this model to five neutrino-associated Seyfert galaxies, we successfully reproduce the observed TeV neutrino fluxes without violating existing gamma-ray constraints. By integrating over the Seyfert population using X-ray luminosity functions, we further demonstrate that Seyfert galaxies can account for a substantial fraction of the diffuse astrophysical neutrino background in the $10^4-10^5~{\rm GeV}$ energy range.

Figures (5)

• Figure 1: Example proton interaction timescales for NGC 1068. Detailed expressions for the timescale calculations are provided in the main text. The adopted parameters are: $\mathcal{R} = 15$, $\epsilon_{\rm B} = 0.01$, $\eta_{\rm k} = 0.1$, and $v_0 = 0.03c$.
• Figure 2: The all-flavor neutrino and gamma-ray fluxes for NGC 1068 under various parameter combinations, with the proton spectral index fixed at $\Gamma_{\rm p} = 2$ and energy conversion efficiency $\eta_{\rm k} = 0.1$. Red lines (with different linestyles) represent the predicted neutrino fluxes, while black lines denote the corresponding cascade photon fluxes. Blue data points show Fermi-LAT 16-year observations, and purple arrows indicate MAGIC upper limits acciari2019constraints. The pink shaded region marks the neutrino flux detected by IceCube abbasi2025evidence.
• Figure 3: The all-flavor neutrino and gamma-ray SEDs for five neutrino-associated sources: NGC 1068, NGC 4151, NGC 3079, CGCG 420-015, and NGC 7469. Blue points represent the 16-year Fermi-LAT gamma-ray data. The shaded regions indicate the neutrino fluxes detected by IceCube, with data sourced from abbasi2025evidence for NGC 7469, NGC 4151, and CGCG 420-015, and from neronov2024neutrino for NGC 3079.
• Figure 4: Diffuse neutrino and gamma-ray contributions from a population of Seyfert galaxies, assuming the Schwarzschild radius of $R_{\rm s} = 10^{12.5}~\mathrm{cm}$, which corresponds to an SMBH with mass $M_{\rm BH} = 10^7~M_{\odot}$. The extragalactic gamma-ray background (EGB) and isotropic gamma-ray background (IGRB) data are taken from ackermann2015spectrum, while the diffuse neutrino background data are adopted from naab2023measurement.
• Figure 5: Combined disk and corona SEDs for the five neutrino-associated Seyfert nuclei analyzed in this work.