Interpreting Swift and NuSTAR Observations of the Low-Luminosity Active Galactic Nucleus NGC 4278 with Radiatively Inefficient Accretion Flows and Implications for Neutrino Emission

Abstract

We report the first NuSTAR hard X-ray observations of the low-luminosity active galactic nucleus NGC 4278. The source is clearly detected beyond 10 keV with a hard X-ray spectrum consistent with a power law of photon index between 2.2 and 2.5 without evidence for a high-energy cutoff. The X-ray flux is low compared to the active state in 2021, but exhibits variability by a factor of ~2 on a timescale of a month. We discuss the origin of the hard X-ray emission and explore its connection to gamma rays and high-energy neutrinos. We explain the X-ray data, including both quiescent and active states, using a radiatively inefficient accretion flow (RIAF) model with a variable accretion rate. We also show that TeV gamma rays cannot escape from the RIAF disk, and very high-energy gamma rays observed in LHAASO are likely to originate from outer regions such as jets and winds, which is consistent with our results favoring a magnetically arrested disk. We also discuss hidden neutrino emission from RIAFs together with possible connections to coronae of active galactic nuclei with standard, radiatively efficient disks.

Figures (9)

• Figure 1: The X-ray spectra of NGC 4278. Quasi-simultaneous spectra measured by Swift-XRT (blue squares) and NuSTAR (dark blue down triangles for FPMA and light blue plus signs for FPMB) on January 13, 2025, show a moderate flux level. The Swift-XRT spectrum measured during the LHAASO-reported active state (pink circles) shows an X-ray active state that is more prominent above 2 keV. The NuSTAR spectrum (orange up triangles for FPMA and gold cross signs for FPMB) measured on December 6, 2024, shows a low-flux state. The flux points are corrected for the Galactic neutral hydrogen absorption.
• Figure 2: (Top) The X-ray light curve of NGC 4278 between 2020 and 2025, showing an active X-ray flux state during the LHAASO-reported active periods. (Bottom) A zoomed-in view of the variability during the X-ray campaign in 2024 and 2025, showing a low to moderate flux state with variability by a factor of $\sim2$ over timescales of weeks to months.
• Figure 3: The broadband SED of NGC 4278. The active state SED is measured by Swift-XRT (pink circles), Fermi-LAT (red diamonds) Bronzini2024, and LHAASO (tomato red circles) Cao2024_NGC4278. The "quiescent" NuSTAR SED was measured on December 6, 2024. The "moderate" X-ray SED was measured quasi-simultaneously by Swift-XRT and NuSTAR on January 13, 2025 (see Sec. \ref{['subsec:xspec']} and \ref{['subsec:var']}). The archival SEDs are taken from the ASI Space Science Data Center (SSDC) SED Builder Stratta2011.
• Figure 4: Left: SEDs with varying $\alpha$ for constant values of $\beta$ and $\dot{m}$. Middle: Same with $\beta$ for constant values of $\alpha$ and $\dot{m}$. Right: Same with $\dot{m}$ for constant values of $\alpha$ and $\beta$. The archival and X-ray data points have the same legends as fig. \ref{['fig:sed']}. The purple and black data points are from Nemmen1 and Banerjee2019 respectively.
• Figure 5: Multi-zone SED from different radii with $R=(10-40)~R_S$, compared to a contribution from each radius. The archival and X-ray data points have the same legends as Fig. \ref{['fig:sed']}. For comparison, the LINER average SED is shown from Nemmen2 (dashed line).