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 \simeq 1.5 arcmin .
Early Fermi -LAT observations revealed a significant decade-timescale brightening of NGC 1275 at GeV photon energies , with a flux about seven 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 cut-off around an observed photon energy of \varepsilon _ { \gamma } = 42.2 \pm 19.6 GeV , with an average flux of F _ { \gamma } = ( 2.31 \pm 0.13 ) \times 10 ^ { -7 } ph cm ^ { -2 } s ^ { -1 } and a power-law photon index , \Gamma _ { \gamma } = 2.13 \pm 0.02 .
The largest \gamma -ray flaring event was observed in April–May 2009 and was accompanied by significant spectral variability above \varepsilon _ { \gamma } \gtrsim 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 \varepsilon _ { \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 .