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

Abstract

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 jet deceleration or wind expansion. We further show that future MeV and very-high-energy $\gamma$-ray observations can discriminate between the leptonic and lepto-hadronic scenarios. Although the neutrino flux from NGC 4278 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 ($n_{\rm L,0}$) corrected for the TeV duty cycle ($\Delta T_{\rm TeV}/T$, the fraction of a LLAGN's lifetime spent in a TeV-emitting phase), $n_{\rm L,0}(\Delta T_{\rm TeV}/T) \sim 10^{-5}~\rm Mpc^{-3}$, as inferred from the LHAASO detection.