Origin of the Very High Energy {\gamma}-rays in the Low-luminosity Active Galactic Nucleus NGC 4278

TL;DR

This study suggests that the low-luminosity AGN NGC 4278 may produce very high energy gamma rays during active phases, with its spectral energy distribution modeled using a leptonic approach indicating unique magnetic field conditions.

Contribution

It provides the first analysis linking VHE gamma-ray emission to NGC 4278's active nucleus and employs a one-zone leptonic model to explain its broadband spectral energy distribution.

Findings

NGC 4278 was in a high-flux X-ray state during gamma-ray activity.

A leptonic model with lower magnetic fields fits NGC 4278's SED.

NGC 4278's properties are more similar to LINERs and Seyferts than BL Lacs.

Abstract

NGC 4278, a Low-luminosity active galactic nucleus (AGN), is generally classified as a low-ionization nuclear emission line region (LINER). Recently, it has been reported to be associated with a very high energy $\gamma$-ray source 1LHAASO J1219+2915 in the first Large High Altitude Air Shower Observatory source catalog. However, no associated counterpart has been detected by analyzing the data collected by the Large Area Telescope on board the Fermi Gamma-ray Space Telescope. By analyzing its X-ray observation data from Swift-XRT, we find NGC 4278 is in a high-flux state on MJD 59546, with the X-ray flux more than one order of magnitude higher than that observed $\sim$ 11.7 year earlier by Chandra. Interestingly, this Swift-XRT observation was conducted during the active phase of the $\gamma$-ray source 1LHAASO J1219+2915. We propose that the detection of VHE $\gamma$-rays from NGC 4278 may be attributed to the presence of an active nucleus in its center. To reproduce the spectral energy distribution (SED) of NGC 4278, we employ a one-zone leptonic model, typically used for fitting broadband SEDs of BL Lacs, and find that a smaller magnetic field strength is required than that of typical TeV BL Lacs. Furthermore, NGC 4278 exhibits significantly lower luminosity in both radio and TeV bands when compared with typical TeV BL Lacs. In the radio-luminosity vs. Eddington-ratio plane, NGC 4278 shows greater similarity to Seyfert galaxies and LINERs rather than BL Lacs; however, it still roughly follows the extension towards lower luminosity seen in BL Lacs.