Kataoka, J. ; Stawarz, Ł. ; Cheung, C. C. ; ...Cavazzuti, E. ; et al.
May - 2010
ISSN : 0004-637X ;
journal : The Astrophysical Journal

Issue : 1
type: Article Journal

We report on a detailed investigation of the high-energy ³-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 ³-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 C1.5 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 ³-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 µ³ = 42.2 ± 19.6 GeV, with an average flux of F ³ = (2.31 ± 0.13) × 107 photons cm2 s1 and a power-law photon index, “³ = 2.13 ± 0.02. The largest ³-ray flaring event was observed in 2009 April-May and was accompanied by significant spectral variability above µ³ s 1-2 GeV. The ³-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 ³-ray source was detected at the very end of the observation, with the observed energy of µ³ = 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 ³-ray spectral evolution of NGC 1275 in the context of ³-ray blazar sources in general.

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