TeV γ -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, active galactic nucleus 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 leptohadronic emission from subrelativistic 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 γ -ray observations can discriminate between the leptonic and leptohadronic scenarios. Although the neutrino flux from NGC 4278 predicted by the wind model is too low to be detected with current neutrino observatories, a leptohadronic wind scenario can account for the PeV diffuse neutrino background when adopting a local LLAGN density ( n L,0 ) corrected for the TeV duty cycle (Δ T TeV / T , the fraction of a LLAGN’s lifetime spent in a TeV-emitting phase), n L,0 (Δ T TeV / T ) ∼ 10 −5 Mpc −3 , as inferred from the LHAASO detection.