致密河外射电源的不透明度及其对射电-光学参考系对准的影响
Opacity in compact extragalactic radio sources and its effect on radio-optical reference frame alignment
摘要：
无线电和光学天体参考系的精确对准需要详细了解可能导致在不同光谱带测量的同一物体的位置之间的偏移的物理因素。致密河外喷流的不透明度（由于同步加速器自吸收和外部自由-自由吸收）是产生这种偏移的关键物理现象之一，这种效应在射电天文学中是众所周知的（“核心移位”）。我们测量了29个明亮的紧凑型河外射电源使用甚长基线干涉测量（VLBI）在2.3和8.6 GHz的样品中的核心位移。我们报告了这些测量的结果，并估计遥远的类星体的射电和光学位置之间的平均位移将是0.1-0.2质量的顺序。这种偏移超过了GAIA和SIM的位置精度。我们提出了两种可能的方法来仔细研究和纠正这种影响，以准确地对准无线电和光学位置。两种方法都涉及确定要用于将无线电和光学参考系捆绑在一起的对象的主参考样本。
英文摘要：
Accurate alignment of the radio and optical celestial reference frames requires detailed understanding of physical factors that may cause offsets between the positions of the same object measured in different spectral bands. Opacity in compact extragalactic jets (due to synchrotron self-absorption and external free-free absorption) is one of the key physical phenomena producing such an offset, and this effect is well-known in radio astronomy ("core shift"). We have measured the core shifts in a sample of 29 bright compact extragalactic radio sources observed using very long baseline interferometry (VLBI) at 2.3 and 8.6 GHz. We report the results of these measurements and estimate that the average shift between radio and optical positions of distant quasars would be of the order of 0.1-0.2 mas. This shift exceeds positional accuracy of GAIA and SIM. We suggest two possible approaches to carefully investigate and correct for this effect in order to align accurately the radio and optical positions. Both approaches involve determining a Primary Reference Sample of objects to be used for tying the radio and optical reference frames together.
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