Fermi Large Area Telescope Detection of Gamma Rays from the NGC 6251 Radio Lobe

We report the detection of extended γ -ray emission from lobes in the radio galaxy NGC 6251 using observation data from the Fermi Large Area Telescope. The maximum likelihood analysis results show that a radio morphology template provides a better fit than a pointlike source description for the observational data at a confidence level of 8.1 σ , and the contribution of lobes constitutes more than 50% of the total γ -ray flux. Furthermore, the γ -ray energy spectra show a significant disparity in shape between the core and lobe regions, with a curved log-parabola shape observed in the core region and a power-law form observed in the lobes. Neither the core region nor the northwest (NW) lobe displays significant flux variations in the long-term γ -ray light curves. The broadband spectral energy distributions of the core region and the NW lobe can be explained with a single-zone leptonic model. The γ -rays of the core region are due to the synchrotron-self-Compton process, while the γ -rays from the NW lobe are interpreted as inverse Compton emission of the cosmic microwave background.