IDV源的空间VLBI偏振测量：来自VSOP的教训和VSOP2的前景
Space VLBI polarimetry of IDV sources: Lessons from VSOP and prospects for VSOP2
摘要：
空间VLBI观测对于类星体致密发射区的定位和成像至关重要，而致密发射区与日内光变现象密切相关。在这里，我们报告的VSOP观测两个突出的IDV源，BL Lac对象S5 0716+714。为了监测其短期的变化，这些来源与VSOP在5 GHz的几个偏振敏感的实验中观察到，在时间上分开一天到六天，在2000年秋季。利用Effelsberg 100米射电望远镜的同期流量密度测量，直接比较了单碟IDV与VLBI图像的变化。在0716+714中观察到总强度和线性偏振的明显IDV行为。对VLBI观测数据的分析表明，这些变化主要发生在VLBI核心分量0716+714的内部。VLBI地面阵列图像和VSOP图像与单碟测量结果一致，都显示VLBI核心的总通量密度下降约20 mJy，线偏振下降约5 mJy。在射流中没有发现变化。根据变化时间尺度，我们估计一个源的大小为几微弧秒，亮度温度超过10^15 K。无论VLBI芯中的IDV是内源性的还是外源性的，通过VLBI芯的模型拟合得到的下限T_B > 2 × 10^12 K。我们的研究结果表明，未来的VSOP 2观测应伴随着一个单一的碟监测，不仅要区分源-外在和源-内在的影响，但也允许一个适当的校准和超高分辨率VSOP 2图像的解释。
英文摘要：
To locate and image the compact emission regions in quasars, which are closely connected to the phenomenon of IntraDay Variability (IDV), space VLBI observations are of prime importance. Here we report on VSOP observations of two prominent IDV sources, the BL Lac objects S5 0716+714. To monitor their short term variability, these sources were observed with VSOP at 5 GHz in several polarisation sensitive experiments, separated in time by one day to six days, in autumn 2000. Contemporaneous flux density measurements with the Effelsberg 100m radio telescope were used to directly compare the single dish IDV with changes of the VLBI images. A clear IDV behaviour in total intensity and linear polarization was observed in 0716+714. Analysis of the VLBI data shows that the variations are located inside the VLBI core component of 0716+714. In good agreement with the single-dish measurements, the VLBI ground array images and the VSOP images, both show a decrease in the total flux density of ~20 mJy and a drop of ~5 mJy in the linear polarization of the VLBI core. No variability was found in the jet. From the variability timescales we estimate a source size of a few micro-arcseconds and brightness temperatures exceeding 10^15 K. Independent of whether the interpretation of the IDV seen in the VLBI core is source intrinsic or extrinsic a lower limit of T_B > 2x10^12 K is obtained by model fitting of the VLBI-core. Our results show that future VSOP2 observations should be accompanied by a single dish monitoring not only to discriminate between source-extrinsic and source-intrinsic effects but to allow also a proper calibration and interpretation of ultra-high resolution VSOP2 images.
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