标题： 活动星系核喷流中VLBI测量的核心移动效应的偏差 显示英文标题

标题： A bias in VLBI measurements of the core shift effect in AGN jets

摘要： 布兰福德和科尼格模型预测了活动星系核（AGN）喷流中表观不透明喷流基底（即核心）的位置会随频率变化。 这种效应可以通过射电干涉测量观测到，并广泛用于推断最内层喷流区域的参数和结构。 通常通过将高斯模板拟合到干涉测量可见度上来估计射电核心的位置。 这会导致一个模型近似误差，即可以通过具有实际喷流模型的观测模拟来检测和评估的偏差。 为了评估这一偏差，我们基于从真实 VLBI 通量密度受限样本推导出的参数网格上评估的 AGN 喷流模型构建了一组人造源样本，并在 2.3、8.1 和 15.4 GHz 创建模拟 VLBI 数据集。 我们发现，相对于真实喷流顶点，核心位置的偏移量通常被高估。 该偏差通常与核心偏移的随机误差相当，并且对于具有大视开口角的喷流，可以达到两倍。 这种观测偏差主要取决于真实核心偏移与图像分辨率之比。 这意味着从核心偏移测量中推断出的磁场、核心径向距离和喷流速度都被高估了。 我们提出了一种方法来考虑这一偏差。 显示英文摘要

摘要： The Blandford and K\"{o}nigl model of AGN jets predicts that the position of the apparent opaque jet base - the core - changes with frequency. This effect is observed with radio interferometry and is widely used to infer parameters and structure of the innermost jet regions. The position of the radio core is typically estimated by fitting a Gaussian template to the interferometric visibilities. This results in a model approximation error, i.e. a bias that can be detected and evaluated through simulations of observations with a realistic jet model. To assess the bias, we construct an artificial sample of sources based on the AGN jet model evaluated on a grid of the parameters derived from a real VLBI flux-density-limited sample and create simulated VLBI data sets at 2.3, 8.1 and 15.4 GHz. We found that the core position shifts from the true jet apex are generally overestimated. The bias is typically comparable to the core shift random error and can reach a factor of two for jets with large apparent opening angles. This observational bias depends mostly on the ratio between the true core shift and the image resolution. This implies that the magnetic field, the core radial distance and the jet speed inferred from the core shift measurements are overestimated. We present a method to account for the bias.