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

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.

💡 Research Summary

The authors investigate the origin of the very‑high‑energy (VHE; > 1 TeV) γ‑ray emission detected by LHAASO from the low‑luminosity active galactic nucleus (LL AGN) NGC 4278. By assembling a multi‑wavelength data set—including VLBA/VLA radio, Swift‑XRT X‑ray, Fermi‑LAT GeV γ‑ray, and LHAASO TeV γ‑ray observations—as well as IceCube neutrino data, they construct broadband spectral energy distributions (SEDs) for two distinct activity states: a flaring phase (L₀.₁–10 TeV ≈ 3 × 10⁴¹ erg s⁻¹) and a quasi‑quiescent phase (≈ 4.3 × 10⁴⁰ erg s⁻¹). Radio imaging yields a modest Doppler factor δ≈2.6–2.8, indicating mildly relativistic jets viewed at a small angle. Swift‑XRT analysis shows a photon index Γ≈2.11 and an unabsorbed 0.3–10 keV flux of 1.4 × 10⁻¹² erg cm⁻² s⁻¹, confirming a low‑flux, quasi‑steady X‑ray state during the LHAASO observation window. The GeV band provides only upper limits (TS≈8.5), while the VHE spectrum is well measured by LHAASO, displaying a power‑law with EBL attenuation.

To interpret the SED, the authors employ the time‑dependent, one‑zone leptohadronic code AMES. The model solves coupled transport equations for electrons, protons, and photons, including synchrotron, synchrotron‑self‑Compton (SSC), external inverse‑Compton (EIC), photomeson, Bethe‑Heitler, and γ‑γ absorption processes. Primary particles are injected with a power‑law (index s≈2.0–2.3) and an exponential cutoff; the emission region is a spherical blob of radius R′≈10¹⁶ cm, magnetic field B′≈0.1–1 G, and escape time t_esc≈R′/c. Acceleration is parametrized by t_acc≈η ε′/(e B′ c) with η≥1 (Bohm limit).

Two scenarios are examined:

1. Standard SSC – The target photon field is the observed weak X‑ray emission. Because the X‑ray luminosity is low (L_X≈10⁴⁰ erg s⁻¹), the synchrotron photon density inside the blob is insufficient to up‑scatter electrons to the observed TeV flux. The SSC model underpredicts the VHE flux by ~70 % unless an unusually high Doppler factor (δ ≳ 5) or an implausibly large electron energy density is assumed. Such a high δ conflicts with the radio‑derived value and would require a jet power far exceeding the independent estimate of P_jet≈(1–2) × 10⁴² erg s⁻¹.

2. External Inverse‑Compton (EIC) with RIAF seed photons – The authors adopt seed photons from a radiatively inefficient accretion flow (RIAF) surrounding the supermassive black hole (M_BH≈3 × 10⁸ M_⊙). The RIAF produces a soft IR–optical photon field (ν ≈ 10¹³–10¹⁶ Hz) that, when scattered by the relativistic electrons in the jet, can generate the observed TeV emission with a modest Doppler factor (δ≈2.6–3) and a jet power consistent with radio estimates. The EIC model reproduces both the X‑ray and VHE points simultaneously, and the required magnetic field (B′≈0.2 G) and electron injection power are physically reasonable.

Within the leptohadronic framework, the authors also compute the expected high‑energy neutrino flux from photomeson interactions of accelerated protons. Assuming that 0.1 % of the Eddington luminosity (L_Edd≈3.9 × 10⁴⁶ erg s⁻¹) is channeled into relativistic protons, the EIC‑quiescent model yields the highest muon‑neutrino event rate, N_{ν_μ} ≈ 0.001 over a 15‑year IceCube exposure. This rate is well below IceCube’s current sensitivity (90 % upper limit ≈ 8 × 10⁻¹³ TeV⁻¹ s⁻¹ cm⁻²) and consistent with the non‑detection reported in the paper.

The paper concludes that the VHE γ‑rays from NGC 4278 are most naturally explained by external inverse‑Compton scattering of RIAF photons rather than a pure SSC process, given the modest Doppler factor and jet power constraints. The predicted neutrino flux is low but not negligible; future deeper IceCube observations or next‑generation neutrino telescopes could test the leptohadronic component. The authors emphasize that coordinated multi‑messenger monitoring (radio, X‑ray, GeV/TeV γ‑rays, and neutrinos) will be essential to disentangle particle acceleration and radiation mechanisms in LL AGNs like NGC 4278.