γ-RAY SPECTRAL EVOLUTION OF NGC 1275 OBSERVED WITH FERMI LARGE AREA TELESCOPE

We report on a detailed investigation of the high-energy gamma-ray emission from NGC 1275, a well-known radio galaxy hosted by a giant elliptical located at the center of the nearby Perseus cluster. With the increased photon statistics, the center of the gamma-ray-emitting region is now measured to be separated by only 0.46 arcmin from the nucleus of NGC 1275, well within the 95% confidence error circle with radius similar or equal to 1.5 arcmin. Early Fermi Large Area Telescope (LAT) observations revealed a significant decade-timescale brightening of NGC 1275 at GeV photon energies, with a flux about 7 times higher than the one implied by the upper limit from previous EGRET observations. With the accumulation of one year of Fermi-LAT all-sky-survey exposure, we now detect flux and spectral variations of this source on month timescales, as reported in this paper. The average > 100 MeV gamma-ray spectrum of NGC 1275 shows a possible deviation from a simple power-law shape, indicating a spectral cutoff around an observed photon energy of epsilon(gamma) = 42.2 +/- 19.6 GeV, with an average flux of F-gamma = (2.31 +/- 0.13) x 10(-7) photons cm(-2) s(-1) and a power-law photon index, Gamma(gamma) = 2.13 +/- 0.02. The largest gamma-ray flaring event was observed in 2009 April-May and was accompanied by significant spectral variability above epsilon(gamma) greater than or similar to 1-2 GeV. The gamma-ray activity of NGC 1275 during this flare can be described by a hysteresis behavior in the flux versus photon index plane. The highest energy photon associated with the gamma-ray source was detected at the very end of the observation, with the observed energy of epsilon(gamma) = 67.4 GeV and an angular separation of about 2.4 arcmin from the nucleus. In this paper we present the details of the Fermi-LAT data analysis, and briefly discuss the implications of the observed gamma-ray spectral evolution of NGC 1275 in the context of gamma-ray blazar sources in general.