Rapid Gamma-Ray Variability of NGC 1275.

We report on a detailed analysis of the γ-ray light curve of NGC 1275 using the Fermi Large Area Telescope data accumulated during 2008–2017. Major γ-ray flares were observed in 2015 October and 2016 December/2017 January when the source reached a daily peak flux of , achieving a flux of within 3 hr, which corresponds to an apparent isotropic γ-ray luminosity of . The most rapid flare had an e-folding time as short as hr, which had never been previously observed for any radio galaxy in γ-ray band. Also, γ-ray spectral changes were observed during these flares: in the flux versus photon index plane, the spectral evolution follows correspondingly a counterclockwise and a clockwise loop inferred from the light curve generated by an adaptive binning method. On 2016 December 30 and 2017 January 1 the X-ray photon index softened () and the flux increased nearly ∼3 times as compared with the quiet state. The observed hour-scale variability suggests a very compact emission region (), implying that the observed emission is most likely produced in the subparsec-scale jet if the entire jet width is responsible for the emission. During the active periods, the γ-ray photon index hardened, shifting the peak of the high-energy spectral component to , making it difficult to explain the observed X-ray and γ-ray data in the standard one-zone synchrotron self-Compton model. [ABSTRACT FROM AUTHOR]