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 Large Area Telescope (LAT) region around NGC 4278, limited to the LHAASO campaign conducted from 2021 March to 2022 October. A statistically significant emission (∼4.3 σ ) was revealed, spatially consistent with the radio position of NGC 4278 and the LHAASO source. The Fermi-LAT source is detected above 8 GeV, exhibiting a hard spectrum (photon index Γ = 1.3 ± 0.3) and a γ -ray luminosity of L _>100 MeV ≃ 4 × 10 ^41 erg s ^–1 . A serendipitous Swift X-Ray Telescope (XRT) observation of NGC 4278 during the TeV campaign reveals the source in a high state, with a flux ${F}_{0.5-8\,{\rm{ke}}\,{\rm{V}}}={5}_{-2}^{+3}\times 1{0}^{-12}\,{\rm{erg}}\,{{\rm{s}}}^{-1}\,{{\rm{cm}}}^{-2}$ , compatible with 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 can exceed the sensitivity thresholds of GeV and TeV instruments, becoming promising targets for the upcoming Cherenkov Telescope Array Observatory.

Keywords

• Gamma-rays
• X-ray active galactic nuclei
• Low-luminosity active galactic nuclei
• Relativistic jets