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arXiv 0808.2385astro-ph
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非吸收Seyfert 2星系的性质

On the nature of unabsorbed Seyfert 2 galaxies

Murray Brightman, Kirpal Nandra

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10 pages, 5 figures, accepted for publication at MNRAS
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摘要：

我们提出了一个12 μ m的选择6个赛弗特2星系，已被报道为在X射线不吸收的分析。通过比较这些星系在中红外（12 μ m）、光学（[O III]）和硬X射线（2-10 keV）波段的光度，我们发现它们在2-10 keV的X射线波段都是欠亮的。其中四个天体的X射线光谱显示出硬过剩，与严重模糊的X射线成分一致，因此是一个隐藏的核。在这些天体中，较软的X射线可能主要是由一个强的软散射连续体或来自宿主星系的污染，这是造成观察到的未吸收的X射线光谱，并解释了令人不安的低2-10 keV的X射线光度。我们在NGC 4501中用钱德拉观测证实了这一说法，它显示硬X射线发射与原子核一致，与重吸收一致，以及一些污染的软源，这些软源占软发射的大部分。我们指出，这种“康普顿厚”的来源不一定存在铁Ka排放高EW。我们样本中的一个例子是IRASF 01475 -0740，我们知道它必须拥有一个模糊的活动星系核，因为它拥有一个在散射光中看到的HBLR（Tran 2003）。尽管如此，X射线光谱还是比较清晰的，铁的Ka线强度也很适中。如果隐藏的核辐射是由透射的X射线而不是反射的X射线主导的，那么这些观测结果可以被调和，与软散射光或银河系辐射相比，反射的X射线可能很弱。尽管有这些考虑，我们得出结论，NGC 3147和3660可能本质上缺乏BLR，证实了Bianchi等人（2008）对NGC 3147的最新结果。两个X射线光谱都没有显示出隐藏的硬发射的迹象，两个源都表现出X射线的可变性，这使我们相信我们正在直接观察原子核。

英文摘要：

We present an analysis of six 12 um selected Seyfert 2 galaxies that have been reported to be unabsorbed in the X-ray. By comparing the luminosities of these galaxies in the mid-IR (12um), optical ([O III]) and hard X-ray (2-10 keV), we show that they are all under-luminous in the 2-10 keV X-ray band. Four of the objects exhibit X-ray spectra indicative of a hard excess, consistent with a heavily obscured X-ray component and hence a hidden nucleus. In these objects the softer X-rays may be dominated by a strong soft scattered continuum or contamination from the host galaxy, which is responsible for the unabsorbed X-ray spectra observed, and accounts for the anomalously low 2-10 keV X-ray luminosity. We confirm this assertion in NGC4501 with a Chandra observation, which shows hard X-ray emission coincident with the nucleus, consistent with heavy absorption, and a number of contaminating softer sources which account for the bulk of the softer emission. We point out that such 'Compton thick' sources need not necessarily present iron Ka emission of high EW. An example in our sample is IRASF01475-0740, which we know must host an obscured AGN as it hosts a HBLR seen in scattered light (Tran 2003). The X-ray spectrum is nonetheless relatively unobscured and the iron Ka line only moderate in strength. These observations can be reconciled if the hidden nuclear emission is dominated by transmitted, rather than reflected X-rays, which can then be weak compared to the soft scattered light or galactic emission. Despite these considerations, we conclude that NGC3147 & 3660 may intrinsically lack a BLR, confirming the recent results of Bianchi et al. (2008) for NGC3147. Neither X-ray spectrum shows signs of hidden hard emission and both sources exhibit X-ray variability leading us to believe we are viewing the nucleus directly.

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