Neutrinos from super-Eddington Seyfert galaxies

Technical Summary: Neutrinos from Super-Eddington Seyfert Galaxies

Problem Statement
Multimessenger observations have identified Seyfert galaxies as promising sources of high-energy neutrinos, notably NGC 1068, which exhibits a significant neutrino flux but a suppressed very-high-energy (VHE) gamma-ray counterpart. This "neutrino–gamma-ray gap" suggests that the neutrino production region is embedded in an environment opaque to high-energy photons. While theoretical models often focus on sub-Eddington active galactic nuclei (AGN) with hot coronae, the physical conditions required to efficiently produce neutrinos while simultaneously absorbing gamma rays in super-Eddington regimes remain under-explored. Specifically, the interplay between supercritical accretion flows, radiation-driven winds, and particle acceleration mechanisms in these dense environments requires a dedicated investigation.

Methodology
The authors develop a one-zone theoretical model for a supermassive black hole (SMBH) of mass $M_{BH} = 10^7 M_\odot$ accreting at super-Eddington rates ($\dot{m} > 1$). The model incorporates the following physical components:

1. Disk and Wind Structure: The accretion flow is modeled using the supercritical disk solution of Fukue (2004) and Akizuki and Fukue (2006). Inside a critical radius ($r_{crit}$), the flow becomes geometrically and optically thick, launching powerful radiation-driven winds. The wind forms a dense, opaque funnel wall, while the inner polar region remains a low-density channel.
2. Particle Acceleration: Relativistic particles (protons and electrons) are accelerated via magnetic reconnection within a magnetically confined region close to the SMBH ($z_{acc} \approx 10 r_g$). The magnetic flux is estimated based on a sub-Magnetically Arrested Disk (sub-MAD) scenario, appropriate for radio-quiet Seyfert galaxies.
3. Radiative Processes and Cooling: The model computes energy loss rates for primary particles and secondary species (pions, muons, kaons). Key processes include:
   • Protons: Synchrotron radiation, inelastic $pp$ collisions, and $p\gamma$ interactions with the intense thermal photon fields of the disk and wind.
   • Electrons: Synchrotron radiation and Inverse Compton (IC) scattering off disk and wind photons.
   • Secondaries: Decay chains of charged pions and kaons into neutrinos, and neutral pion decay into gamma rays.
4. Opacity and Spectral Energy Distributions (SEDs): The authors calculate the optical depth for gamma rays due to $\gamma\gamma$ absorption (pair production) in the thermal radiation fields of the disk and wind, as well as $\gamma N$ interactions with the wind matter. This determines the escaping gamma-ray spectrum.
5. Neutrino Flux Calculation: The steady-state particle distributions are solved using a transport equation. The resulting neutrino emissivities are integrated to calculate the flux at Earth, accounting for flavor oscillations over astrophysical distances.

Two representative configurations are analyzed:

• Model A: Weakly super-Eddington ($\dot{m}=3$) with low magnetization ($\beta_{disk}=0.5$).
• Model B: Moderately super-Eddington ($\dot{m}=10$) with moderate magnetization ($\beta_{disk}=5$).

Key Contributions and Results

• Hidden Hadronic Accelerators: The study demonstrates that super-Eddington AGN cores can act as efficient hadronic accelerators. The intense photon field from the supercritical disk and the surrounding wind facilitates efficient $p\gamma$ interactions, producing a copious neutrino flux peaking in the 10–100 TeV range.
• Gamma-Ray Suppression: The dense wind and disk radiation fields create a high optical depth for gamma rays. The model shows that gamma rays produced in the inner core are strongly attenuated via $\gamma\gamma$ and $\gamma N$ absorption, particularly for observers viewing the system at high inclinations. This naturally explains the coexistence of bright neutrino emission and faint or undetected VHE gamma-ray counterparts.
• Particle Spectra and Maximum Energies:
  • In Model A, protons reach maximum energies of $\sim 1$ PeV; in Model B, they reach $\sim 10$ PeV.
  • Electrons are limited to $\sim 200$ GeV due to efficient IC cooling.
  • The neutrino spectrum is dominated by the $p\gamma$ channel at high energies, with a secondary contribution from $pp$ interactions at lower energies.
• Detectability:
  • Model A yields neutrino fluxes below the current sensitivity of IceCube.
  • Model B produces fluxes ($\sim 3 \times 10^{-10}$ GeV cm$^{-2}$ s$^{-1}$) that approach the sensitivity limits of current observatories (IceCube) and are well within the reach of next-generation detectors like IceCube-Gen2 and KM3NeT.
• Application to NGC 7469: The model is applied to the nearby Seyfert 1 galaxy NGC 7469, a source recently associated with two high-energy neutrino events (IC 220424A and IC 230416A).
  • Assuming a super-Eddington accretion rate ($\dot{m}=10$) and a hard proton injection index ($\Gamma \approx 1.5-2$), the model reproduces the observed neutrino flux and spectral hardness.
  • The associated gamma-ray emission is predicted to be suppressed below Fermi-LAT detection limits due to the dense environment, consistent with non-detections.
  • The model suggests that proton synchrotron radiation, rather than electron synchrotron, may dominate the non-thermal radio emission in such systems due to the severe IC cooling of electrons.

Significance and Claims
The paper posits that super-Eddington Seyfert galaxies, particularly those undergoing transient intense accretion episodes (such as those triggered by tidal disruption events), are plausible "hidden" neutrino sources. The primary significance of this work lies in providing a self-consistent physical framework that explains:

1. Efficient Neutrino Production: Via $p\gamma$ interactions in the intense radiation field of a supercritical disk.
2. Strong Gamma-Ray Attenuation: Via absorption in the dense, radiation-driven wind and disk photon fields.
3. Observational Consistency: The model offers a natural explanation for the neutrino excesses observed in NGC 1068 and NGC 7469 without requiring relativistic jets, which are typically absent in radio-quiet Seyferts.

The authors conclude that these systems represent a distinct class of multimessenger sources where the electromagnetic counterpart is heavily reprocessed, while the neutrino signal escapes essentially unattenuated. Future multimessenger campaigns combining neutrino data with optical, UV, X-ray, and gamma-ray observations are identified as crucial for testing this scenario and constraining the physics of supercritical accretion flows.