北天大样本IDV巡天观测研究

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题名	北天大样本IDV巡天观测研究
作者	刘俊
学位类别	博士
答辩日期	2015
授予单位	中国科学院大学
授予地点	中国科学院新疆天文台
导师	刘祥 ; Thomas P. Krichbaum
关键词	活动星系核耀变体大样本统计光变日变化射电连续谱星际介质星际闪烁
其他题名	A Large-Sample Survey for IDV in the Northern Sky
学位专业	理学
中文摘要	光变是活动星系核（Active Galactic Nuclei，AGN）最重要的观测特征之一。AGN在整个可观测电磁波段表现出光变，时标从分钟到数十年不等。通常将光变时标为天量级或小于一天的光变称为短时标光变（日变化，IDV）。观测表明有大约25~50% 的平谱射电类星体和BL Bac 天体表现出IDV 现象。IDV的物理机制至今仍是悬而未决的问题，主要集中在其内禀起源和外禀起源上。两种解释均有观测证据的支持。但是越来越多的观测表明，银河系内星际介质对河外AGN辐射电磁波的星际闪烁是IDV的主要成因。我们对北天的446 个致密平谱射电源进行了大样本的IDV 巡天观测。该巡天观测旨在通过统计的手段，揭示致密射电源的IDV 起因。巡天项目的主要科学目标在于：（1）大样本IDV搜寻；（2）检验IDV光变的依赖性；（3）检验IDV与弥散/离散星际介质分布的相关性。这些也是本论文的主要内容。我们共计探测到166 个IDV 源，IDV的探测率为37%。Fermi 源的IDV 探测率要明显高于非Fermi 源，表明该样本中射电源存在某种射电射线之间的相关性。我们对IDV的光变进行了依赖性分析——检验IDV光变幅度是否依赖AGN的物理参数如其光学对应体的类型、流量密度、谱指数、射线噪度，红移、银纬分布等。我们发现IDV 光变幅度显著地依赖这些物理参量，具体表现为：（1）流量密度与光变幅度存在反相关。这种反相关同时表现在数量和强度上；（2） IDV更趋向出现在反转谱的射电源中；（3）光变表现出强烈的银纬依赖。低银纬处有更大的IDV探测率，且低银纬射电源有更强的光变；（4）低红移射电源的平均具有更高的光变幅度。其中（1）（2）（3）条非常符合星际闪烁模型的预期。此外，我们分析了银河系弥散辐射对IDV光变的影响。我们对H_(alpha) 、IRAS100 微米以及银河系旋转测量强度的分布与样本射电源的光变强度进行了相关分析，均发现了相关性。这些结果更加强化了我们之前的结论。进一步，我们认为银河系内的局部离散星际介质分布应该能反映到我们的观测数据上。由此我们分析了Loop III（射电圈3）周围的光变幅度分布，发现在Loop III 外圈附近的源光变更加剧烈，但是由于这种源的数目很少，结果的置信度不是太高。接着，我们分析了银河系的本地星际云的分布对IDV的影响，蒙特卡洛模拟表明，在这些星际云中，观测到的光变强度都超过了样本的整体预期，这说明本地星际云作为可能的闪烁介质，对我们观测到的IDV光变有明显的影响。第4 章我们分析并讨论了BL Lac 天体0925+504 在4.8 GHz 的快速光变。该源在4 历元的观测中有时标为0:5 1:1 天的快速光变现象。我们分别用折射式闪烁和弱闪烁模型研究了该源的结构函数。我们对模型拟合的参数进行了分析和比较，发现该源的辐射区非常致密，而且星际介质距离观测者非常近。此外我们在该源中发现了可能的周年调制效应。我们的结果说明观测到的0925+504的快速光变主要是由距观测者~110 pc 的闪烁屏的闪烁导致的。最后一章是对本工作的总结和展望，我们认为，星际闪烁对IDV有普遍的影响，多波段的AGN光变观测和分析将是未来工作的重点。
英文摘要	Among the all characteristics of Active Galactic Nuclei (AGNs), one of the most remarkable is the strong time variability they show on time scales from minutes to dozens of years, over the whole observable electromagnetic spectrum. Variability on time scales of a day or less is classified as short-term variability (also known as intra-day variability, or IDV). Observations show that IDV is detected in a significant fraction ( 25~50% ) in flat-spectrum radio quasars and BL Lac objects. The physical mechanism responsible for IDV remains open for debate, with models involving both source-intrinsic as well as source-extrinsic explanations, and both gain supporting in observations. Evidence has now accumulated to demonstrate that interstellar scintillation (ISS) in our Galaxy is the dominant cause of such variability. We performed a large-sample northern IDV survey with 446 compact flat-spectrum radio sources , aiming at discovering the origin of IDV in compact radio sources. The scientific objectives of the survey are, (i) searching for IDV sources in a large sample; (ii) verifying the dependence of IDV; (iii) testing the correlation of IDV with the distribution of diffuse/discrete interstellar medium (ISM). All of the above mentioned topics constitute the main contents of this thesis. A total of 166 sources are found to exhibit IDV, which gives an IDV detection rate of 37% . The IDV detection rate is remarkably higher in Fermi sources than in non-Fermi sources, indicating possible correlations between the radio and -ray emission for the sources in our sample. The dependence of variability–any possible relationship between the degree of variability and source parameters such as the optical identification, source flux density, spectral index, -ray loudness, galactic latitude and redshift, etc. are checked. We found significant variability dependence on these parameters, (i) source flux density is inversely proportional to the variability amplitude; (ii) IDV are tend to exhibit in sources with inverted spectrum; (iii) strong galactic latitude dependence is found, where the detection rate is higher at lower galactic latitude region and sources show stronger variability with lower galactic latitudes. (iv) sources with low redshifts show averagely larger amplitude of variability. Among these, the first three findings are consistent with the ISS model. Furthermore, we study the effect of galactic diffuse emission on IDV. We tested the correlation of variability amplitude with the all-sky distribution of H emission, IRAS 100 micron emission and the galactic rotation measure, and found correlations in all these maps, which confirms our previous conclusions. Moreover, we suspect that the existence of local discrete interstellar medium may contribute to our data. For this reason, we investigated how the IDV distribution depend on the location of Loop III. The result shows that sources which lie in the outer region of Loop III exhibit stronger variability. However due to lack of sufficient source number in this region, a statistically confident conclusion cannot be addressed. Then the correlation of source variability with the distribution of the local interstellar clouds (LICs) are verified. Monte-Carlo simulations show that the observed variability in the LICs are higher than expected. The result suggesting the LICs are possible medium responsible for the observed variability. Rapid variability of the BL BLac object 0925+504 at 4.8 GHz is analysed and and discussed in chapter 4. The source shows rapid variability on time scales of 0.5 ~1.1 days in the four epochs of observations. The structure functions (SF) are investigated with both refractive and weak interstellar scintillation (RISS/WISS) models analytically. Parameters obtained with these models are quantitatively compared, suggesting that the emission region of IDV is remarkably compact and the responsible ISM lies very close to the observer. Furthermore possible evidence of annual modulation of the variability time scales is detected in this source. Our findings indicate that the observed rapid variability in 0925+504 is predominantly caused by a scattering screen located along the line of sight to the source, at a distance of~110 pc to the observer. Finally the summary and future plans are given in the last chapter. We conclude that ISS contribute commonly to IDV sources, and our future work will concentrate on multi-wavelength observations and analysis on AGN variability.
语种	中文
学科主题	天体物理
内容类型	学位论文
源URL	[http://ir.xao.ac.cn/handle/45760611-7/645]
专题	研究生新疆天文台_射电天文研究室
推荐引用方式 GB/T 7714	刘俊. 北天大样本IDV巡天观测研究[D]. 中国科学院新疆天文台. 中国科学院大学. 2015.