利用VLBI研究活动星系核中的吸收
文献类型：学位论文
| 作者 | 蔡宏兵 |
| 学位类别 | 博士 |
| 答辩日期 | 2007-05-18 |
| 授予单位 | 中国科学院上海天文台 |
| 授予地点 | 上海天文台 |
| 导师 | 沈志强 |
| 关键词 | 活动星系核 喷流 吸收 VLBI |
| 其他题名 | Investigation of the absorption in Active Galactic Nuclei using VLBI |
| 中文摘要 | In this dissertation, we mainly discuss the observationl effects caused by the synchrotron self-absorption (SSA) and free-free absorption (FFA). We study PKS~0528+134 by using VLBA observations at five frequencies at 2001.64. We also study 3C~66A by using VLBA observations at 2.3, 8.4, and 22.2~GHz at six epochs. The radio radiation from AGN is likely to be absorbed more or less by the absorbing medium in AGN and the medium between AGN and the observer. The absorption depresses the radiation intensity we receive, but it also gives us a new access to study the environment in and outside of AGN. For the radio continuum, the main absorption mechanisms are SSA and FFA. Synchrotron photons may be absorbed through the interaction with low energy non-thermal electrons; this process is called SSA. The energy of thermal electrons may increase as a result of radio photons being absorbed when radio photons interact with thermal electrons, this process is called FFA. Some observing effects caused by SSA and FFA are quite similar. Both SSA and FFA can induce the inverse of the radio continuum at low frequencies. For a uniform synchrotron source, the spectral index at low frequencies is 2.5 for SSA, but for FFA, it is steeper than 2.5. The position of VLBI core is defined as the position where the optical depth is 1. The higher the observing frequency, the deeper region will be detected, so the positions of the VLBI core at high frequencies will be closer to the jet apex than at low frequencies, i.e., there is a positional offset of the VLBI core at different frequencies. This positional offset has been observed by many VLBI observations, but we cannot discriminate which is the main mechanism responsible for such an offset, SSA or FFA. Another similar observational effect caused by SSA and FFA is the polarization angle variation of the radio wave when at different frequencies. The polarization angle difference at high frequencies and low frequencies due to SSA is $\sim 90^{\circ}$. The plasma screen for the FFA can cause the change of the polarization angle at different frequencies too; this change is proportional to the square of the observing wavelength, which is known as the Faraday rotation. Due to these similar effects caused by SSA and FFA, people are searching the methods to discriminate them. We performed VLBA observations of PKS~0528+134 at 2.3, 5.0, 8.4, 15.4, and 22.2~GHz, and find that PKS~0528+134 has a structure extending north-east and the jet has two bendings at about 0.3 and 1.4~mas from the core. We find a new component n2, which may be related to the radio burst at 1999.5. By extrapolating the proper motion of component a, we estimate its ejection time of 1991.94, but we cannot ascertain which radio burst is related to its ejection. We also find a phenomenon of position-angle regression; i.e., the position angle of the jet component gradually decreases with its increasing distance from the core until 25$^{\circ}$. The quasi-simultaneous VLBA observations at five frequencies enable us to study the spectral distribution of the jet components without the effect of flux-density variation. We identify k as the core because of its flat spectrum ($\alpha=0.30\pm 0.08$) and the spectral inverse at low frequencies. We try to use the helical model to describe the bending of the jet; the kinetic parameters obtained from the helical model are consistent with ones obtained from our data. This helical model can also explain the position-angle regression. We proposed to observe PKS~0528+134 with VLBA at eight frequencies for studying the absorption mechanism in the core region and the spectral distribution of the jet components. The proposal has been approved. Combining the previous VLBI observations, we can also study the kinematics of the jet components in PKS~0528+134. We present results of VLBA observations of 3C~66A at three frequencies during six epochs. We find from our VLBA maps that there are two bendings at 1.2 and 4 mas from the core. We identify the brightest k component as the core from our simultaneous spectral data which show an inverse. Combining the existing VLBI data at 5 GHz, we use the SSA model to fit the core spectrum at epoch 2001.48 for the first time, and obtain the best-fit optically-thin spectral index of $-0.08$ with a maximum flux density 0.66 Jy at the turnover frequency of 6.52 GHz. We do not detect any obvious proper motions in jet components during our shortly-spanned observing epochs. We give a possible identification of our jet components with the jet components in previous VLBI observations. The positional offset of component d relative to the core at 2.3 and 8.4~GHz is consistently seen at all six epochs. We also study the light curves of 3C~66A at different frequencies, and find that the simultaneous radio burst at different frequencies may be due to the rotation of the jet axis, then the change of our viewing angle to the jet emission, and thus the change of flux density induced by the change of Doppler factor. |
| 英文摘要 | 本文主要讨论了同步自吸收 (SSA) 和自由自由吸收 (FFA) 所引起的观测效应. 利用美国国家射电天文台的VLBA 阵列在5个频率上在历元2001.64对射电源PKS~0528+134进行了观测研究. 我们还利用VLBA阵列在频率2.3, 8.4, 22.2~GHz在6个历元上对3C~66A进行了观测研究. 活动星系核中射电辐射在到达我们观测者的过程中, 总是不同程度地被吸收. 吸收降低了到达我们观测者的辐射强度, 但是吸收也为我们研究活动星系核的内、外部物理环境提供了一种新的途径. 在射电连续谱波段我们常遇到的吸收有两种: SSA和FFA. SSA是同步辐射光子和非热电子发生碰撞, 光子被吸收而使非热电子能量升高的物理过程; FFA是光子和热电子发生碰撞, 光子被吸收而使热电子能量升高的物理过程. 它们都和频率有关, 频率越低, 吸收越严重. SSA和FFA有许多相似的观测效应. 它们都会引起低频谱的倒转, 对SSA来说, 如果同步辐射源是均匀的, 低频谱指数为2.5; 对FFA来说, 低频谱比SSA更陡. VLBI核的位置一般定义为光深为1的位置, 由于频率越高, 看得越深, 所以高频VLBI核离喷流顶点近, 低频VLBI核离喷流顶点远, 从而在不同频率上观测时核的位置会偏移. 核的位置在不同频率上发生偏移已经被许多VLBI观测所证明, 但现在仍然不清楚这种偏移主要是由SSA引起的还是FFA引起的. SSA和FFA另外一个相似的观测效应是偏振位置角的变化. SSA可使同步辐射的偏振位置角改变将近90$^{\circ}$, 也就是说在高频观测到的偏振位置角与在低频观测到的偏振位置角几乎相差90$^{\circ}$. 但是引起FFA的带磁场的等离子体屏也会使辐射的偏振位置角发生旋转, 即所谓的法拉第旋转效应. 法拉第旋转效应引起的偏振位置角的改变与观测波长的平方成正比, 而SSA引起的偏振位置角的变化与波长没有此依赖关系. 由于SSA和FFA有这些相似的观测效应, 所以人们一直在寻找区分这两种吸收机制的方法. 我们利用VLBA对PKS~0528+134在2.3, 5.0, 8.4, 15.4, 22.2~GHz上进行了观测, 发现其有一个向东北方向延伸的结构, 在离核0.3和1.4~mas的地方喷流发生了弯曲. 发现了一个新的成份n2, 可能与1999.5年的射电爆发有关. 通过外推成份a的自行, 发现a的喷出时间大概为1991.94, 但是根据单天线的光变曲线, 我们不能确定哪一次射电爆发和a的喷出有关. 我们也发现当喷流成份刚出来时其位置角是比较大的, 然后随着喷流成份离核的距离增大, 位置角逐渐的减小, 直到25$^{\circ}$, 我们把它称为位置角回归现象. 对PKS~0528+134在5个频率上的准同时观测使我们能够研究喷流成份的谱分布, 而不必考虑光变的影响. 由于成份k有着较平的谱 ($\alpha=0.30\pm 0.08$) 且其在低频有倒转, 我们把它确定为核. 我们尝试用螺旋喷流模型去描述喷流的弯曲轨迹. 从这个螺旋模型得到的运动学参数和我们通过观测得到的是一致的. 螺旋模型还能解释位置角回归现象. 为了研究PKS~0528+134中核的吸收机制及喷流成份的谱分布，我们申请到了8个频率上的VLBA观测. 通过这些观测, 并且结合以前的观测, 我们也可以研究喷流成份的运动学. 我们给出了6个历元上对3C~66A的多频VLBA观测结果. 从VLBA图上发现在离核1.2和4~mas的地方喷流发生弯曲. 通过研究VLBI成份的谱, 我们证认k为核. 结合以前5~GHz上的VLBI数据, 我们第一次用SSA模型去拟合核在2001.48上的谱, 得到最佳拟合谱指数为$-0.08$, 峰值流量密度为0.66~Jy, 倒转频率6.52~GHz. 由于我们的观测时间覆盖较短, 并没有发现喷流成份有明显的自行. 通过外推以前VLBA观测中喷流成份的自行, 给出了我们的喷流成份和他们这些成份之间的证认. 我们也发现了喷流成份d在2.3和8.4~GHz上在6个历元上都有位置偏移. 通过对3C~66A的光变分析, 我们认为在多个频率上观测到同时爆发可能是由于喷流轴的旋转引起我们的视线与喷流轴夹角的变化, 从而多普勒增亮因子的变化导致流量密度的变化. |
| 语种 | 中文 |
| 公开日期 | 2011-07-01 |
| 页码 | 133 |
| 源URL | [http://119.78.226.72//handle/331011/14630]  |
| 专题 | 上海天文台_中国科学院上海天文台学位论文 |
| 推荐引用方式 GB/T 7714 | 蔡宏兵. 利用VLBI研究活动星系核中的吸收[D]. 上海天文台. 中国科学院上海天文台. 2007. |
入库方式： OAI收割
来源：上海天文台
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