标题： 相位参考VLBI观测中的相干损失 显示英文标题

标题： On the coherence loss in phase-referenced VLBI observations

摘要： 上下文：相位参考是一种在射电干涉测量中标准的校准技术，特别适用于检测接近干涉仪灵敏度极限的弱源。 然而，变化的大气效应和相关器模型的不准确性可能会影响相位参考图像，导致源通量密度和位置的错误估计。 尚未进行过对相位参考中信号去相干的系统观测研究及其在图像平面上的影响。 目标：我们系统地研究了在不同频率和不同校准源与目标分离情况下，甚长基线干涉测量（VLBI）观测中的信号相干性如何受到影响。 获得的结果应有助于正确解释许多使用VLBI的相位参考观测。 方法：我们在相位参考模式下，在8.4和15 GHz频率下观测了一组13个强源（S5极帽样本），使用32种不同的校准源/目标组合，覆盖1.5到20.5度之间的角分离。 我们获得了相位参考图像，并研究了动态范围和峰值通量密度如何依赖于观测频率和源分离度。 结果：我们获得了相位参考图像的动态范围和峰值通量密度作为频率和校准源分离度的函数。 我们将结果与之前报道的模型和经验方程进行了比较。 结论：相位参考图像的动态范围在所有频率和所有源分离度下都受到大气的强烈限制。 限制动态范围与校准源与目标分离度的正弦值成反比。 我们还发现，相对于自校准图像获得的峰值通量密度，随着源分离度的增加而减少。 显示英文摘要

摘要： Context: Phase referencing is a standard calibration technique in radio interferometry, particularly suited for the detection of weak sources close to the sensitivity limits of the interferometers. However, effects from a changing atmosphere and inaccuracies in the correlator model may affect the phase-referenced images, leading to wrong estimates of source flux densities and positions. A systematic observational study of signal decoherence in phase referencing, and its effects in the image plane, has not been performed yet. Aims: We systematically studied how the signal coherence in Very-Long-Baseline-Interferometry (VLBI) observations is affected by a phase-reference calibration at different frequencies and for different calibrator-to-target separations. The results obtained should be of interest for a correct interpretation of many phase-referenced observations with VLBI. Methods: We observed a set of 13 strong sources (the S5 polar cap sample) at 8.4 and 15 GHz in phase-reference mode, with 32 different calibrator/target combinations spanning angular separations between 1.5 and 20.5 degrees. We obtained phase-referenced images and studied how the dynamic range and peak flux density depend on observing frequency and source separation. Results: We obtained dynamic ranges and peak flux densities of the phase-referenced images as a function of frequency and separation from the calibrator. We compared our results with models and phenomenological equations previously reported. Conclusions: The dynamic range of the phase-referenced images is strongly limited by the atmosphere at all frequencies and for all source separations. The limiting dynamic range is inversely proportional to the sine of the calibrator-to-target separation. We also find that the peak flux densities, relative to those obtained with the self-calibrated images, decrease with source separation.