四个ICRF2源的位置/结构稳定性
The Position/Structure Stability of Four ICRF2 Sources
摘要：
利用VLBA（43、23和8.6 GHz）和VERA（23 GHz）的相位参考，对国际天体参考系（ICRF）星表中的4个近距离射电源进行了为期一年的观测。目标是确定无线电核心的稳定性，并评估与ICRF中的位置相关的结构效应。虽然这四个源在8.6 GHz时是紧凑的，但在43 GHz时具有0.3-mas分辨率的VLBA图像显示它们都由几个组件组成。每个源中的一个组成部分被确定为无线电核心，使用以下一些或全部发射特性：紧凑性，光谱指数，在扩展发射区域末端的位置，以及在天空中的静止状态。在观测期间，四个无线电核心之间的相对位置恒定为0.02 mas-在23和43 GHz的源之间获得的相位参考位置精度-这表明一旦确定了无线电核心，它在天空中保持静止以达到此精度。其他无线电组件在两个四个来源有可检测的运动在无线电喷流方向。23和43 GHz的VLBA图像与VLBA 8.6 GHz的图像和ICRF的位置的比较表明，一些ICRF的位置是由一个移动的喷流分量占主导地位，因此，他们可以从无线电核心位移高达0.5质量，也可能反映喷流分量的运动。未来的天体测量工作，以确定一个更准确的类星体参考系在23和43 GHz和VLBI 2010项目进行了讨论，并支持VLBA或EVN观测ICRF源在43 GHz的建议，以确定内部结构的来源。无线电（ICRF）和盖亚的光学框架之间的未来合作进行了讨论。
英文摘要：
Four close radio sources in the International Celestial Reference Frame (ICRF) catalog were observed using phase referencing with the VLBA at 43, 23 and 8.6 GHz, and with VERA at 23 GHz over a one year period. The goal was to determine the stability of the radio cores, and to assess structure effects associated with positions in the ICRF. Although the four sources were compact at 8.6 GHz, the VLBA images at 43 GHz with 0.3-mas resolution showed that all were composed of several components. A component in each source was identified as the radio core using some or all of the following emission properties: compactness, spectral index, location at the end of the extended emission region, and stationary in the sky. Over the observing period, the relative positions between the four radio cores were constant to 0.02 mas---the phase referencing positional accuracy obtained at 23 and 43 GHz among the sources---suggesting that once a radio core is identified, it remains stationary in the sky to this accuracy. Other radio components in two of the four sources had detectable motion in the radio jet direction. Comparison of the 23 and 43 GHz VLBA images with the VLBA 8.6 GHz images and the ICRF positions suggests that some ICRF positions are dominated by a moving jet component; hence, they can be displaced up to 0.5 mas from the radio core, and may also reflect the motion of the jet component. Future astrometric efforts to determine a more accurate quasar reference frame at 23 and 43 GHz and from the VLBI2010 project are discussed, and supporting VLBA or EVN observations of ICRF sources at 43 GHz are recommended in order to determine the internal structure of the sources. A future collaboration between the radio (ICRF) and the optical frame of Gaia is discussed.
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