Physical origin of very-high-energy gamma rays from the low-luminosity active galactic nucleus NGC 4278 and implications for neutrino observations

Abstract

Relativistic jets in active galactic nuclei (AGNs) are known to accelerate particles to extreme energies, yet the physical origin of very-high-energy (VHE) emission from low-luminosity AGNs (LL AGNs) remains unclear. NGC 4278, a local LLAGN, has recently been identified as a VHE source following detections by LHAASO. In this study, we present a multi-wavelength and multi-messenger analysis to investigate the physical origin of this emission. Swift-XRT monitoring reveals a quasi-quiescent state characterized by a low X-ray flux. Modeling the broadband spectral energy distribution with the leptohadronic code AMES, we find that a standard one-zone synchrotron self-Compton (SSC) model underpredicts the VHE flux by $\sim$70% due to the insufficient target photon density provided by the weak X-ray emission, unless a high Doppler factor ($\delta \gtrsim 5$) is invoked. Alternatively, an external inverse-Compton (EIC) scenario-scattering seed photons from a radiatively inefficient accretion flow (RIAF)-successfully reproduces the broadband spectral energy distribution with a modest jet power and Doppler factor. We further explore the neutrino production within a leptohadronic framework. The predicted muon neutrino event rate is highest in the EIC quiescent model, reaching $N_{\nu_{\mu}} \sim 0.001$ for a 15-year IceCube observation (assuming 0.1% of the Eddington luminosity is partitioned into high-energy protons). Future multi-messenger observations are essential to unveil the details of the high-energy processes of NGC 4278.