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致密河外射电源的不透明度及其对天体物理和天体测量研究的影响

Opacity in compact extragalactic radio sources and its effect on astrophysical and astrometric studies

Y. Y. Kovalev, A. P. Lobanov, A. B. Pushkarev, J. A. Zensus

备注：
11 pages, 8 figures, 2 tables, accepted for publication in the Astronomy and Astrophysics; minor corrections to the manuscript are made
发表信息：
A&A 483 (2008), 759-768
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摘要：

在秒差距尺度的射电喷流（喷流窄端的一个紧凑明亮的发射区域）中，“核心”的视位置取决于观测频率，这是由于同步加速器的自吸收和外部吸收。虽然提供了一个工具，探测在核心附近的物理条件，这种依赖性提出了问题，天体测量研究使用紧凑的无线电源。我们研究了在秒差距尺度喷流中用甚长基线干涉仪（VLBI）观测到的核心位置随频率的变化（核心位移）。我们讨论了相关的物理，以及它对射电天体测量的影响和天体测量参考对象的射电和光学位置之间的联系。我们在2002年和2003年使用VLBI观测在2.3 GHz（13 cm）和8.6 GHz（4 cm）频带成像的277个射电源样本中搜索核心位移。通过将核心位置参考到光学薄射流特征来测量核心偏移，所述光学薄射流特征的位置预期不会随频率而改变。我们在这里提出的结果为29个选定的活动星系核（AGN）明亮的独特的VLBI喷流功能，可用于差分测量，并允许强大的测量的转变。在这些活动星系核中，在2.3和8.6 GHz之间测得的核心位移的幅度达到1.4 mas，与样本的中值为0.44 mas。核耀斑导致了这种转变的时间变化。无线电（4cm）和光学（6000埃）频带之间的平均偏移估计为大约0.1质量，并且为了提供无线电-光学参考系连接的所需精度，应当将其考虑在内。这可以通过多频VLBI测量来实现。（节略）

英文摘要：

The apparent position of the "core" in a parsec-scale radio jet (a compact, bright emitting region at the narrow end of the jet) depends on the observing frequency, owing to synchrotron self-absorption and external absorption. While providing a tool probing physical conditions in the vicinity of the core, this dependency poses problems for astrometric studies using compact radio sources. We investigated the frequency-dependent shift in the positions of the cores (core shift) observed with very long baseline interferometry (VLBI) in parsec-scale jets. We discuss related physics, as well as its effect on radio astrometry and the connection between radio and optical positions of astrometric reference objects. We searched for the core shift in a sample of 277 radio sources imaged at 2.3 GHz (13 cm) and 8.6 GHz (4 cm) frequency bands using VLBI observations made in 2002 and 2003. The core shift was measured by referencing the core position to optically thin jet features whose positions are not expected to change with frequency. We present here results for 29 selected active galactic nuclei (AGN) with bright distinct VLBI jet features that can be used in differential measurements and that allow robust measurements of the shift to be made. In these AGN, the magnitude of the measured core shift between 2.3 and 8.6 GHz reaches 1.4 mas, with a median value for the sample of 0.44 mas. Nuclear flares result in temporal variability of the shift. An average shift between the radio (4 cm) and optical (6000 Angstrom) bands is estimated to be approximately 0.1 mas, and it should be taken into account in order to provide the required accuracy of the radio-optical reference frame connection. This can be accomplished with multi-frequency VLBI measurements... (abridged)

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