2011MNRAS.413.2785B

We report on our study of the high-energy γ-ray emission from the Fanaroff–Riley type I (FR I) radio galaxy NGC 1275, based on 2 yr of observations with the Fermi Large Area Telescope (LAT) detector. Previous Fermi studies of NGC 1275 had found evidence for spectral and flux variability on monthly time-scales during the first year of Fermi-LAT observations. This variability is also seen in the larger 2-yr data set, during which we observe a large γ-ray flare (2010 June–August). The increased photon statistics from this large flare have allowed the discovery of flux variability from NGC 1275 on the time-scales of days. The largest flux variation observed during this flare being a factor of ∼ 3 from 1 day to the next and a resultant e-folding rise time of 1.51±0.2 d. The 2-yr-averaged E > 100 MeV γ-ray spectrum is adequately described by a power-law spectrum, with a photon index, Γ, of 2.09±0.02, and a resultant integrated flux of F_γ= (2.2±0.1) {x} 10^–7 photon/cm2/s. While no hysteresis was observed in the photon index–flux (Γ_γ versus F_γ) parameter space, there was obvious `harder-when-brighter' behaviour observed during the large γ-ray flare. Furthermore, during this large flare, NGC 1275 appeared to migrate from the FR I radio galaxy to the BL Lac object region of the photon index–luminosity (Γ_γ versus L_γ) paramater space. In this paper, we present details of our Fermi-LAT analysis of NGC 1275, including a brief discussion on its implications for γ-ray blazar sources.