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

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