TeV Gamma-Rays from the Low-Luminosity Active Galactic Nucleus NGC 4278: Implications for the Diffuse Neutrino Background

This work investigates the origin of the TeV emission detected by the Large High Altitude Air Shower Observatory (LHAASO) from NGC 4278, a galaxy hosting a low-luminosity active galactic nucleus (LLAGN). Considering two plausible scenarios, AGN jets and winds, we model the X-ray, GeV, and TeV emission during both TeV-low (quasi-quiet) and TeV-high (active) states. The spectral energy distributions can be explained either by single-zone leptonic emission from moderately relativistic jets or by lepto-hadronic emission from sub-relativistic winds. The best-fit parameters suggest that the transition from the quasi-quiet to the active state may be driven jointly by an enhanced accretion rate and the expansion of jets or winds. We further show that future MeV and very-high-energy $γ$-ray observations can discriminate between the {leptonic and lepto-hadronic scenarios}. Although the neutrino flux from NGC1068 predicted by the wind model is too low to be detected with current neutrino observatories, a lepto-hadronic wind scenario can account for the PeV diffuse neutrino background when adopting a local LLAGN density corrected for the TeV duty cycle, $n_{\rm L,0}(ΔT_{\rm TeV}/T) \sim 10^{-5}\rm Mpc^{-3}$, as inferred from the LHAASO detection.

💡 Research Summary

This paper investigates the origin of the TeV γ‑ray emission recently detected by LHAASO from the low‑luminosity active galactic nucleus (LLAGN) NGC 4278. The source exhibits two distinct states: a “quasi‑quiet” baseline and an “active” phase during which the TeV flux increases by a factor of ~7, accompanied by simultaneous enhancements in Swift‑XRT X‑ray and Fermi‑LAT GeV γ‑ray bands. The authors model the multi‑wavelength spectral energy distributions (SEDs) for both states using two physically motivated scenarios.

In the jet scenario, a compact spherical blob (radius R′_b) moves with a bulk Lorentz factor Γ_b≈3–4 and contains a magnetic field B′_b of a few milligauss. Relativistic electrons are injected with a power‑law spectrum (s≈2.2) and a high‑energy cutoff γ′_e,max. Synchrotron radiation accounts for the radio‑to‑X‑ray component, while synchrotron‑self‑Compton (SSC) reproduces the GeV–TeV emission. By fitting the data, the authors find that the transition from the quiet to the active state can be described by an expanding blob (R′_b increases), a modest decrease in Γ_b and B′_b, and a rise in the electron injection power L′_e and γ′_e,max. This leptonic jet model successfully reproduces the observed SEDs but does not generate high‑energy neutrinos because protons are not included.

The wind scenario assumes a sub‑relativistic outflow that dissipates its energy at a radius R_diss. Both electrons and protons are injected with power‑law spectra (s=2). Electrons produce low‑energy synchrotron emission, while protons interact with ambient photons (pγ) to create pions; the decay of secondary particles yields X‑ray–γ‑ray photons and PeV‑scale neutrinos. Best‑fit parameters indicate that during the active state the dissipation radius expands to ~8×10^16 cm, the proton luminosity rises to ~4.6×10^44 erg s⁻¹, the magnetic field modestly strengthens to ~0.6 G, and the maximum proton energy reaches ~190 PeV. The model naturally explains the X‑ray/γ‑ray SED and predicts a neutrino flux that, while too low for detection from a single source, can contribute significantly to the diffuse IceCube neutrino background when the local LLAGN density (corrected for the TeV duty cycle, n_L,0≈10⁻⁵ Mpc⁻³) is taken into account.

A key result is that, after correcting for the TeV duty cycle inferred from LHAASO (ΔT_TeV/T≈10⁻⁴), the lepto‑hadronic wind population could account for ~10–30 % of the observed PeV diffuse neutrino flux. The authors also discuss observational discriminants: future MeV‑GeV missions (e‑ASTROGAM, AMEGO) and very‑high‑energy facilities such as CTA can probe the predicted MeV bump from secondary cascades and the γ‑γ attenuation signature that is unique to the wind model.

Overall, the work demonstrates that LLAGNs like NGC 4278 can produce TeV γ‑rays via either moderately relativistic jets or sub‑relativistic, lepto‑hadronic winds. While the jet model is sufficient for the electromagnetic data, the wind model provides a unified explanation for both photons and high‑energy neutrinos, highlighting LLAGNs as potentially important contributors to the cosmic neutrino background. Future multi‑messenger observations will be essential to distinguish between these scenarios and to assess the role of low‑luminosity AGN in the high‑energy universe.