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arXiv 0801.2104astro-ph
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对NGC6251的长期X射线监测:喷流主导射电星系的证据

Long-Term X-Ray monitoring of NGC6251: Evidence for a jet-dominated radio galaxy

M. Gliozzi, I. E. Papadakis, R. M. Sambruna

摘要:

利用RXTE对X射线明亮的FRI射电星系NGC 6251进行了为期一年的首次X射线监测观测。本研究的主要目标是阐明X射线的起源,通过研究与时间分辨和通量选择光谱学相结合的独立于模型的方法的光谱变异性。主要结果如下:1)在整个监测周期中,NGC 6251处于相对高通量状态。2)在20-30天的时间尺度上,通量持续变化,波动量级为~2。3)当硬度比对平均计数率作图时,有证据表明,随着源变亮,光谱硬化;这一发现得到了通量选择光谱分析的证实。4)分数的变化似乎是更明显的硬能带(5-12千电子伏)比软的(2.5-5千电子伏)。5)2个月的平均和通量限制的能量谱是适当的幂律拟合。铁的K α线在统计学上是不需要的,尽管由于线等效宽度的上限很高,不能排除强铁线的存在。NGC 6251的光谱变化行为与塞弗特星系中观察到的典型趋势不一致,与耀变体的相似性导致支持在RXTE监测活动期间以喷气为主的情况。然而,不能排除来自盘-日冕系统的可能贡献。

英文摘要:

We present the first X-ray monitoring observations of the X-ray bright FRI radio galaxy NGC6251 observed with RXTE for 1 year. The primary goal of this study is to shed light on the origin of the X-rays, by investigating the spectral variability with model-independent methods coupled with time-resolved and flux-selected spectroscopy. The main results can be summarized as follows: 1) Throughout the monitoring campaign, NGC6251 was in relatively high-flux state. 2) The flux persistently changed with fluctuations of the order of ~2 on time scales of 20-30 days. 3) When the hardness ratio is plotted against the average count rate, there is evidence for a spectral hardening as the source brightens; this finding is confirmed by a flux-selected spectral analysis. 4) The fractional variability appears to be more pronounced in the hard energy band (5-12 keV) than in the soft one (2.5-5 keV). 5) 2-month averaged and flux-limited energy spectra are adequately fitted by a power law. A Fe Kalpha line is never statistically required, although the presence of a strong iron line cannot be ruled out, due to the high upper limits on the line equivalent width. The inconsistency of the spectral variability behavior of NGC6251 with the typical trend observed in Seyfert galaxies and the similarity with blazars lead support to a jet-dominated scenario during the RXTE monitoring campaign. However, a possible contribution from a disk-corona system cannot be ruled out.

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