Hadronic origin of the very high-energy gamma-ray emission from the low-luminosity AGN in NGC 4278

TL;DR

The paper investigates the hadronic origin of the very high-energy gamma rays from the LLAGN NGC 4278 by combining ALMA CO(2-1) observations with a diffusion-based CR transport model. CR protons accelerated in a radiatively inefficient accretion flow diffuse into a central molecular cloud, producing gamma rays and neutrinos via $pp$ interactions; the fiducial model underpredicts the LHAASO spectrum, but a scenario with a historically higher accretion rate and reduced CR diffusion can reproduce the 1–10 TeV data. HI gas contributes little to the gamma-ray flux, while the predicted neutrino flux lies below IceCube-Gen2 sensitivities; secondary radiation from pion-decay electrons/positrons could yield detectable X-ray signatures in future, providing a critical test of the hadronic scenario. Overall, the study links the VHE emission to the molecular gas environment around a LLAGN and highlights the roles of past activity and CR confinement in shaping gamma-ray outputs with potential multi-messenger and multi-wavelength tests.

Abstract

The Large High Altitude Air Shower Observatory has detected very high-energy (VHE) gamma rays from NGC 4278, which is known to host a low-luminosity active galactic nucleus (AGN). Having only very weak radio jets, the origin of its VHE gamma rays is unclear. In this paper we first show that NGC 4278 has a massive molecular cloud surrounding the nucleus by analyzing data taken with the Atacama Large Millimeter/submillimeter Array. We then assume that cosmic ray protons are accelerated in a radiatively inefficient accretion flow around the supermassive black hole, which diffuse into the molecular cloud and produce gamma rays and neutrinos via $pp$ interactions. We model the gamma-ray spectra and find that the observations can be explained by such hadronic processes if the AGN activity was higher in the past than at present, and the diffusion coefficient in the molecular cloud is appreciably smaller than in the Milky Way interstellar medium. We also show that although the high-energy neutrinos co-produced with the gamma rays are unlikely to be detectable even with IceCube-Gen2, the accompanying synchrotron X-ray emission due to pion-decay secondary electrons and positrons may be detectable in the future, providing a valuable test of our hadronic model.

Figures (7)

• Figure 1: CO emission from NGC 4278. From left to right: integrated intensity map (moment 0), velocity center distribution map (moment 1) of the $\mathrel{ \vcenter{\m@th\f@size4 \ialign{$$\cr >\crcr{ } \sim\crcr}}} 3\sigma$ CO emission, and velocity dispersion map (moment 2) of the $\mathrel{ \vcenter{\m@th\f@size4 \ialign{$$\cr >\crcr{ } \sim\crcr}}} 3\sigma$ CO emission. The position of the AGN is indicated by the cross. The beam size is shown in white at the bottom left of each panel. Alt text: Three color maps, each showing the intensity, velocity center and velocity dispersion of the CO emission.
• Figure 2: Spectra of the hadronic gamma-ray emission model from the molecular cloud in NGC 4278 for $\dot m = 5\times 10^{-6}$ and $\chi=1$ (black solid), $\dot m = 1\times 10^{-3}$ and $\chi=1$ (red dashed), $\dot m = 5\times 10^{-6}$ and $\chi=0.1$ (green dot-dashed), and $\dot m = 5\times 10^{-6}$ and $\chi=0.01$ (green dot-dot-dashed) when $\beta= 3$ and $\zeta= 0.05$. Filled squares and circles show the data from LHAASO 2024ApJ...971L..45C and Fermi 2024ApJS..271...10W, respectively. Alt text: Line graph. The x axis shows the photon energy from $10^{-1}$ to $10^{7}$ giga electron volts. The y axis shows the energy flux from $10^{-13}$ to $10^{-7}$ giga electron volts per squared centimeter per second.
• Figure 3: Model gamma-ray spectra for $\dot m = 5\times 10^{-6}$ and $\chi=1$ (black solid), $\dot m = 5\times 10^{-4}$ and $\chi=1$ (red dashed), and $\dot m = 6\times 10^{-5}$ and $\chi=0.01$ (green dot-dashed) when $\beta = 6,\: \zeta = 0.03$. Otherwise similar to figure \ref{['fig:mol_1']}. Alt text: Line graph similar to figure \ref{['fig:mol_1']}.
• Figure 4: Model gamma-ray spectra from the molecular cloud only (solid) and from both the molecular cloud and the HI gas (dashed) when $\dot m = 5\times 10^{-4}$ and $\chi = 1$. Otherwise similar to figure \ref{['fig:mol_1']}. Alt text: Line graph similar to figure \ref{['fig:mol_1']}.
• Figure 5: The combination of $\dot m$ and $\chi$ that can reproduce the observed gamma-ray data for the molecular cloud only (dashed) and for both the molecular cloud and the HI gas (cyan shaded region). The horizontal line represents the current accretion rate, $\dot m = 5 \times 10^{-6}$. Alt text: Graph of the parameter space. The x axis shows $\chi$ from $0.001$ to $1$. The y axis shows $\dot m$ from $10^{-6}$ to $10^{-2}$.