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

TL;DR

This study investigates the physical origin of very-high-energy gamma rays from the low-luminosity AGN NGC 4278, using multi-wavelength and multi-messenger data including LHAASO VHE detections, Fermi-LAT limits, Swift-XRT spectra, radio jet measurements, and IceCube neutrino constraints. By comparing one-zone SSC, external inverse-Compton (EIC) with a radiatively inefficient accretion flow (RIAF) photon field, and leptohadronic scenarios within a time-dependent transport framework (AMES), the authors find that SSC can reproduce the quasi-quiescent state only if the Doppler factor is high ($δ$ up to ~6), while EIC with RIAF seed photons provides a physically reasonable fit with modest jet power and $δ$ for the quasi-quiescent state. The leptohadronic channel yields neutrino fluxes that are typically below current IceCube sensitivity, though an IceCube detection is not impossible if about $0.1$% of the Eddington luminosity feeds high-energy protons. Overall, the results favor the EIC scenario as the primary driver of VHE emission in the quasi-quiescent state of NGC 4278 and emphasize the critical role of future multi-messenger observations to disentangle high-energy processes in LL AGN jets.

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$70% due to the insufficient target photon density provided by the weak X-ray emission, unless a high Doppler factor ($δ\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 $N_{ν_μ} \sim 0.001$ 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.

Figures (8)

• Figure 1: Multiwavelength light curve. We mainly focus on two time periods. The first period (cyan band) corresponds to the LHAASO flaring state, from MJD 59449 to MJD 59589 LHAASO:2024qzv. The second period (pink band) spans from MJD 60310 to MJD 60796. For the Swift-XRT data, we show the flux per observation. For the Fermi-LAT light curve, we use a time binning of 7 days.
• Figure 2: Broadband SED of NGC 4278. The archival data (small blue points) Younes_2010 correspond to radio and optical observations. The other archival data (small gray points) is obtained from SSDC. The archival X-ray spectrum (gray band) is obtained from Chandra and XMM-Newton observations pellegrini2012agn, where the upper band corresponds to the XMM-Newton observation in 2004 and the lower band to the Chandra observation in 2007. The X-ray spectrum (pink band) lian2024originhighenergygammarays is from Swift–XRT observations in 2021, while the quasi-quiescent X-ray state (blue band) is from Swift–XRT observations in 2024. The GeV data point (red square) bronzini2024fermilatdetectionlowluminosityradio is from Fermi–LAT observations in 2021, and the blue square represent our result which by analyzing Fermi–LAT data from 2024. The VHE data (red and blue dot) LHAASO:2024qzv are obtained from LHAASO observations during an active phase of this source. The brown dash-dotted line is the neutrino 90$\%$ sensitivity of IceCube with a spectral index of 2.0IceCube:2019cia. The purple dash-dotted line is the neutrino 90$\%$ upper limit from this work.
• Figure 3: The broadband SED modeled with one-zone SSC model. The solid red and blue lines represent the SED computed using the 50th percentile (median) of the parameter distributions for the flaring and quasi-quiescent states, respectively. The surrounding red and blue bands indicate the 1 $\sigma$ uncertainty of the fitting parameters.
• Figure 4: Multi-wavelength emission predicted by the EIC model. The solid curves represent the total emission, while the dashed and dash-dotted curves denote the contributions from the RIAF and EIC components, respectively.
• Figure 5: Leptohadronic model from the injection of high-energy protons. The black dashed lines show the all-flavor neutrino spectrum, while the green, blue, red, and purple dashed lines represent the synchrotron emission from primary electrons, IC emission from primary electrons, cascade emission from the photomeson production process, and cascade emission from the Bethe-Heitler pair production process, respectively. Top left: quasi-quiescent state (SSC); Top right: flaring state (SSC); Bottom left: quasi-quiescent state (EIC); Bottom right: flaring state (EIC).