相位参考VLBI观测的相干损失
On the coherence loss in phase-referenced VLBI observations
摘要：
内容：相位参考是无线电干涉测量中的一种标准校准技术，特别适用于检测接近干涉仪灵敏度极限的弱源。然而，来自变化的大气和相关器模型中的不准确性的影响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。系统的观测研究信号退相干相位参考，其影响在图像平面，尚未执行。目的：本文系统地研究了在不同频率和不同定标器-目标距离下，相位参考定标对超长基线干涉测量（VLBI）观测信号相干性的影响。所获得的结果应该是感兴趣的正确解释与VLBI的许多相位参考观测。研究方法：我们观察了一组13个强源（S5极冠样本）在8.4和15 GHz的相位参考模式，32个不同的校准器/目标组合跨越1.5和20.5度之间的角间隔。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。结果如下：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与校准器分离的函数。我们比较了我们的结果与模型和唯象方程先前报道。结论：相位参考图像的动态范围在所有频率和所有源分离下都受到大气的强烈限制。极限动态范围与校准品-目标分离的正弦成反比。我们还发现，峰值通量密度，相对于那些获得的自校准图像，源分离减少。
英文摘要：
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.
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