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

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 ($\mathrm{T}\mathrm{S}=29$) was revealed, spatially consistent with the radio position of NGC 4278 and the LHAASO source. The Fermi-LAT source is detected above $8\mathrm{G}\mathrm{e}\mathrm{V}$, exhibiting a hard spectrum ($\mathrm{\Gamma }=1.3\pm 0.3$) and a $\gamma$-ray luminosity of ${\mathcal{L}}_{>100\mathrm{M}\mathrm{e}\mathrm{V}}\simeq 4\times {10}^{41}\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1}$. A serendipitous Swift-XRT observation of NGC 4278 during the TeV campaign reveals the source in a high-state, with a flux ${\mathcal{F}}_{0.5-8\mathrm{k}\mathrm{e}\mathrm{V}}=5_{-2}^{+3}\times {10}^{-12}\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1}\mathrm{c}{\mathrm{m}}^{-2}$, 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).