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

TL;DR

This study investigates the TeV emission from the LLAGN NGC 4278 detected by LHAASO, testing whether jet SSC emission or lepto-hadronic winds best explains the X-ray, GeV, and TeV data in quasi-quiet and active states. Using two physically motivated frameworks, it fits multiwavelength SEDs and derives parameter trends indicating an expanding emission region with increased particle injection during state transitions. It also assesses the potential contribution of LLAGN winds to the diffuse PeV neutrino background, accounting for TeV-duty cycles and local source density, and finds that winds could be a non-negligible contributor under plausible conditions. The results highlight MeV-band observations as a decisive diagnostic to distinguish leptonic from hadronic processes and motivate future multiwavelength and neutrino-detection efforts to constrain LLAGN contributions to the high-energy neutrino sky.

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 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 NGC~1068 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.

Figures (4)

• Figure 1: Jet SSC scenario: SED fitting for NGC 4278 in the quasi-quiet (left panel) and active (right panel) states. The gray, orange, and purple data points respectively represent the archival radio/infrared/optical data, the Fermi-LAT $\gamma$-ray observations, and the LHAASO TeV $\gamma$-ray measurements, while the blue regions depict the X-ray observations from Swift-XRT. The solid and dashed curves respectively demonstrate the synchrotron and IC components. The cyan regions depict the MeV band. Data sources: LHAASO:2024qzvBronzini:2024vllAnton:2004bmGiroletti:2004bwCardullo:2009zw2010AA...517A..33YPellegrini:2012ws2021ApJ...910..104I.
• Figure 2: Wind lepto-hadronic scenario: SED fitting for NGC 4278 in the quasi-quiet (left panel) and active (right panel) states. The data points have the same meaning with these in Figure \ref{['fig:jet_SEDs']}. In both panels, the cascade photon spectra, the single-flavor neutrino flux, and the external photon spectra are shown as the solid black, solid blue, and dash-dotted gray curves, respectively. Contributions from different interaction components are also displayed. The green and orange solid curves represent the synchrotron and IC radiation from primary electrons. The 10-year flux sensitivities of IceCube IceCube-Gen2:2021rkf and KM3NeT/ARCA230 KM3NeT:2024uhg, at the 90% confidence level, are indicated by the black and purple dash-dotted curves, respectively.
• Figure 3: The cumulative diffuse neutrino spectra (single-flavor) from LLAGN winds that emit at TeV energies. The black and blue points represent the diffuse neutrino fluxes inferred from IceCube high-energy starting events analysis IceCube:2024fxo and six-year shower analysis IceCube:2020acn, respectively. The wind model predictions for $\zeta = 1/6$, $\zeta = 0$, and $\zeta = 1$ are respectively shown as the solid red, dashed green, and dash-dotted curves.
• Figure 4: Corner plots illustrating the posterior parameter distributions for the jet leptonic (left panel) and wind lepto-hadronic (right panel) scenarios in the active (green) and quasi-quiet (blue) states. The contours indicate the $1\sigma$ uncertainties, while the best-fit parameters obtained by minimizing $\chi^2$ are marked by dots.