Astrophys. J., 715, 554-560 (2010/May-3)
γ-ray spectral evolution of NGC 1275 observed with Fermi large area telescope.
KATAOKA J., STAWARZ L., CHEUNG C.C., TOSTI G., CAVAZZUTI E., CELOTTI A., NISHINO S., FUKAZAWA Y., THOMPSON D.J. and McCONVILLE W.F.
Abstract (from CDS):
We report on a detailed investigation of the high-energy γ-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 γ-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 =~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 γ-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 εγ = 42.2±19.6 GeV, with an average flux of Fγ= (2.31±0.13) x 10–7 photons/cm2/s and a power-law photon index, Γγ = 2.13±0.02. The largest γ-ray flaring event was observed in 2009 April-May and was accompanied by significant spectral variability above εγ ≳ 1-2 GeV. The γ-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 γ-ray source was detected at the very end of the observation, with the observed energy of εγ = 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 γ-ray spectral evolution of NGC 1275 in the context of γ-ray blazar sources in general.
galaxies: active - galaxies: individual: NGC 1275 - galaxies: jets - gamma rays: general - radiation mechanisms: non-thermal
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