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arXiv 1004.2352astro-ph.HE
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用Fermi-LAT观测NGC1275的Gamma-ray谱演化

Gamma-ray Spectral Evolution of NGC1275 Observed with Fermi-LAT

J. Kataoka, L. Stawarz, C. C. Cheung, G. Tosti, E. Cavazzuti, A. Celotti, S. Nishino, Y. Fukazawa, D. J. Thompson, W. F. McConville

摘要:

我们报告了对NGC1275的高能γ射线辐射的详细研究。NGC1275是一个著名的射电星系,位于英仙座星系团中心的一个巨大的椭圆星系。随着光子统计的增加,现在测量到伽马射线发射区的中心与NGC1275的核之间的距离仅为0.46 ',完全在半径为1.5'的95%置信误差圆内。早期的Fermi-LAT观测揭示了NGC1275在GeV光子能量下的显著的十年时间尺度增亮,其通量比先前EGRET观测的上限高出约7倍。随着一年的费米-LAT全天巡天曝光的积累,我们现在检测到这个源的通量和光谱变化的月时间尺度,如本文所报道的。NGC 1275的平均值> 100 MeV的伽马射线谱显示出可能偏离简单的幂律形状,表明在观测到的光子能量E = 42.2 +-19.6 GeV附近的谱截止,平均通量F =(2.31 +-0.13)X 10 ^{-7} ph/cm^2/s,幂律光子指数Gamma = 2.13 +-0.02。2009年4月至5月期间观测到最大的伽马射线耀斑事件,并伴随着大于1 - 2 GeV电子的显著光谱变化。NGC1275在这次耀斑期间的伽马射线活动可以用通量与光子指数平面的滞后行为来描述。在观测的最后阶段探测到了与伽马射线源相关的最高能量光子,其观测能量为E = 67.4GeV,与原子核的角距约为2.4 '。在本文中,我们提出的Fermi-LAT数据分析的细节,并简要讨论了所观察到的伽马射线光谱演化的NGC1275的背景下,一般的伽马射线耀变体源的影响。

英文摘要:

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' from the nucleus of NGC1275, well within the 95% confidence error circle with radius ~1.5'. Early Fermi-LAT observations revealed a significant decade-timescale brightening of NGC1275 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 NGC1275 shows a possible deviation from a simple power-law shape, indicating a spectral cut-off around an observed photon energy of E = 42.2+-19.6 GeV, with an average flux of F = (2.31+-0.13) X 10^{-7} ph/cm^2/s and a power-law photon index, Gamma = 2.13+-0.02. The largest gamma-ray flaring event was observed in April--May 2009 and was accompanied by significant spectral variability above E > 1-2 GeV. The gamma-ray activity of NGC1275 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 E = 67.4GeV and an angular separation of about 2.4' 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 NGC1275 in the context of gamma-ray blazar sources in general.

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