2017ApJ...848..111B

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 (2.21±0.26)×10^–6 photon cm^–2 s^–1, achieving a flux of (3.48±0.87)×10^–6 photon cm^–2 s^–1 within 3 hr, which corresponds to an apparent isotropic γ-ray luminosity of ≃3.84×10⁴⁵ erg s^–1. The most rapid flare had an e-folding time as short as 1.21±0.22 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 (Γ_X≃1.75–1.77) and the flux increased nearly ∼3 times as compared with the quiet state. The observed hour-scale variability suggests a very compact emission region (R_γ 5.22×10¹⁴ (δ/4) cm), 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 GeV, making it difficult to explain the observed X-ray and γ-ray data in the standard one-zone synchrotron self-Compton model.