Optical Identification of the ASCA Large Sky Survey
Optical Identification of the ASCA Large Sky Survey
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ASCA 大型巡天系统的光学识别
DOI：
10.1086/308606
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发表时间：
2000
期刊：
影响因子：
--
通讯作者：
W. Voges
中科院分区：
文献类型：
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作者：
M. Akiyama;K. Ohta;T. Yamada;N. Kashikawa;M. Yagi;W. Kawasaki;M. Sakano;T. Tsuru;Y. Ueda;Tadayuki Takahashi;I. Lehmann;G. Hasinger;W. Voges
我们介绍了在ASCA大巡天中探测到的x射线源的光学识别结果。在3.5 σ以上的2 ~ 7 keV波段对34个用SIS探测到的x射线源进行了光谱观测。在2-10 keV波段，通量极限对应于~1 × 10-13 ergs cm-2 s-1。这些源被确定为30个活动星系核（agn），两个星系团和一颗银河系恒星。只有一个来源仍然不明。所有在0.7-10 keV波段具有表观光子指数小于1的硬x射线光谱的x射线源在红移小于0.5时都被识别为窄线或弱宽线agn。这一事实支持了这样一种观点，即窄线和弱宽线AGN的吸收x射线谱使宇宙x射线背景（CXB）谱在硬x射线波段比宽线AGN的谱更硬，这是软x射线波段的主要贡献者。假设它们的本征光谱与宽线AGN（光子指数为1.7的幂律模型）相同，它们的x射线光谱在物体红移处的氢柱密度为log NH(cm-2) = 22-23。另一方面，其他agn的x射线光谱与附近的1型塞弗特星系的x射线光谱一致。在样品中，四个高红移发光宽线agn显示出硬x射线光谱，表观光子指数为1.3±0.3。这种硬度可以用1型塞弗特星系的反射分量来解释。硬x射线光谱也可以用物体红移处log NH(cm-2) = 22-23的吸收来解释，如果我们假设本征光子指数为1.7。硬度的来源尚不清楚。基于对数N - S每个人口的关系,贡献在2 - 10 keV乐队CXB less-absorbed agn的估计是9%(日志NH (cm-2) < 22)包括四个高红移与硬x射线谱宽线agn, 4%吸收agn(<日志NH (cm-2) < 22日23日没有四个硬宽线agn),和1%的星系群通量范围从3×10 - 11尔格cm-2 S - 1到2×10 - 13尔格cm-2 S - 1。如果吸收的agn中包括四种硬宽线agn，则吸收的agn对CXB的贡献估计为6%。在光谱中，我们的样品中没有窄线AGN的高红移发光表兄弟。除4个硬宽线agn外，吸收agn的红移分布限制在z = 0.5以下，而在z = 0.5以上存在15个吸收较少的agn。吸收agn的红移分布表明，在0.5-2红移范围内，或在x射线光度大于1044 ergs -1的范围内，缺乏柱密度为log NH(cm-2) = 22-23的agn。如果四种硬宽线agn的大柱密度是真实的，它们可以弥补x射线吸收发光高红移agn的不足。
We present results of optical identification of the X-ray sources detected in the ASCA Large Sky Survey. Optical spectroscopic observations were done for 34 X-ray sources that were detected with the SIS in the 2-7 keV band above 3.5 σ. The flux limit corresponds to ~1 × 10-13 ergs cm-2 s-1 in the 2-10 keV band. The sources are identified with 30 active galactic nuclei (AGNs), two clusters of galaxies, and one Galactic star. Only one source is still unidentified. All of the X-ray sources that have a hard X-ray spectrum with an apparent photon index of smaller than 1 in the 0.7-10 keV band are identified with narrow-line or weak-broad-line AGNs at redshifts smaller than 0.5. This fact supports the idea that absorbed X-ray spectra of narrow-line and weak-broad-line AGNs make the cosmic X-ray background (CXB) spectrum harder in the hard X-ray band than that of a broad-line AGN, which is the main contributor in the soft X-ray band. Assuming their intrinsic spectra are same as a broad-line AGN (a power-law model with a photon index of 1.7), their X-ray spectra are fitted with hydrogen column densities of log NH(cm-2) = 22-23 at the object's redshift. On the other hand, X-ray spectra of the other AGNs are consistent with that of a nearby type 1 Seyfert galaxy. In the sample, four high-redshift luminous broad-line AGNs show a hard X-ray spectrum with an apparent photon index of 1.3 ± 0.3. The hardness may be explained by the reflection component of a type 1 Seyfert galaxy. The hard X-ray spectra may also be explained by absorption with log NH(cm-2) = 22-23 at the object's redshift, if we assume an intrinsic photon index of 1.7. The origin of the hardness is not clear yet. Based on the log N- log S relations of each population, contributions to the CXB in the 2-10 keV band are estimated to be 9% for less-absorbed AGNs (log NH(cm-2) < 22) including the four high-redshift broad-line AGNs with a hard X-ray spectrum, 4% for absorbed AGNs (22 < log NH(cm-2) < 23, without the four hard broad-line AGNs), and 1% for clusters of galaxies in the flux range from 3 × 10-11 ergs cm-2 s-1 to 2 × 10-13 ergs cm-2 s-1. If the four hard broad-line AGNs are included in the absorbed AGNs, the contribution of the absorbed AGNs to the CXB is estimated to be 6%. In optical spectra, there is no high-redshift luminous cousin of a narrow-line AGN in our sample. The redshift distribution of the absorbed AGNs is limited below z = 0.5 excluding the four hard broad-line AGNs, in contrast to the existence of 15 less-absorbed AGNs above z = 0.5. The redshift distribution of the absorbed AGNs suggests a deficiency of AGNs with column densities of log NH(cm-2) = 22-23 in the redshift range 0.5-2, or in the X-ray luminosity range larger than 1044 ergs s-1, or both. If the large column densities of the four hard broad-line AGNs are real, they could complement the deficiency of X-ray absorbed luminous high-redshift AGNs.