Fermi-LAT detection of the low-luminosity radio galaxy NGC 4278 during the LHAASO campaign

Abstract

We present a study of the high-energy properties of the compact symmetric object NGC 4278, recently associated with a TeV source by the Large High Altitude Air Shower Observatory (LHAASO) collaboration. We conducted a dedicated analysis of a Fermi-LAT region around NGC 4278, limited to the LHAASO campaign conducted from March 2021 to October 2022. A statistically significant emission ($\mathrm{TS} = 29$) was revealed, spatially consistent with the radio position of NGC 4278 and the LHAASO source. The Fermi-LAT source is detected above $8\,\mathrm{GeV}$, exhibiting a hard spectrum ($\Gamma=1.3\pm0.3$) and a $\gamma$-ray luminosity of $\mathcal{L}_{>100 \, \mathrm{MeV}} \simeq 4\times 10^{41} \, \mathrm{erg\,s^{-1}}$. A serendipitous Swift-XRT observation of NGC 4278 during the TeV campaign reveals the source in a high-state, with a flux $\mathcal{F}_{0.5-8\, \mathrm{keV}} = 5_{-2}^{+3} \times 10^{-12} \, \mathrm{erg \, s^{-1}\, cm^{-2}}$, compatible to the highest luminosity level observed in previous Chandra pointings. The high-energy spectral energy distribution of the source and the intense flux variation observed in the X-ray band support a jet origin for the observed radiation. We suggest that the significant enhancement of the high-energy flux observed during the LHAASO campaign is due to a transient, highly energetic perturbation in the jet. The detection of NGC 4278 at both high- and very high-energies opens new frontiers in studying particle acceleration processes. It reveals that even compact, low-power radio galaxies, not just bright blazars, can exceed the sensitivity thresholds of GeV and TeV instruments, becoming promising targets for the upcoming Cherenkov Telescope Array Observatory (CTAO).