Sub-arcsecond imaging with the International LOFAR Telescope I. Foundational calibration strategy and pipeline
Sub-arcsecond imaging with the International LOFAR Telescope I. Foundational calibration strategy and pipeline
复制标题
使用国际 LOFAR 望远镜进行亚角秒成像 I. 基础校准策略和流程
DOI：
10.1051/0004-6361/202140649
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发表时间：
2022
影响因子：
6.5
通讯作者：
Morabito L
中科院分区：
文献类型：
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作者：
Morabito L
国际LOFAR望远镜是一种干涉仪，其观测站遍布欧洲。LOFAR的基线可达~2000 km，具有在低于200 MHz频率下实现亚弧秒分辨率的独特能力。然而，在这种分辨率下处理LOFAR数据在技术上和逻辑上都具有挑战性。到目前为止，只有少数出版物利用了这种能力。在这里，我们提出了一种基于先前使用LOFAR进行的高分辨率工作的校准策略。它是在管道中实现的，主要使用专用的LOFAR软件工具和与LOFAR两米巡天（LoTSS）相同的处理框架。我们概述了校准策略，并讨论了使用LOFAR制定高分辨率成像所固有的特殊挑战，并详细描述了公开可用的管道。我们通过使用P205+55上的管道来演示校准策略，P205+55是一个典型的LoTSS指向，具有8小时的观测和13个国际站。我们执行现场延迟校准，参考现场其他校准器的解决方案，这些校准器的自校准，以及该领域感兴趣的示例方向的成像。我们发现，在这种特殊的场和电离层条件下，色散延迟溶液可以在~1.5°的校准器之间转移，而相溶液转移可以在~1°的校准器之间转移。我们还演示了用场内延迟校准源对天体测量和通量密度尺度的检验。在17个方向上成像，我们发现恢复光束通常为~0.3“×0.2”，尽管在整个5度视野范围内略有变化。我们发现我们可以实现~80 ~ 300 μJy bm−1的图像rms噪声，这取决于到相位中心的距离；在48 MHz带宽下，8 h观测的典型值为~90 μJy bm−1。70%的处理源被检测到，由此我们估计我们应该能够对每个LoTSS指向的大约900个源进行成像。这相当于北方天空中约300万个光源，LoTSS将在未来几年内完全覆盖。为了在高分辨率下对LoTSS进行有效的后处理，未来对校准策略的优化使这一估计成为下限。
The International LOFAR Telescope is an interferometer with stations spread across Europe. With baselines of up to ~2000 km, LOFAR has the unique capability of achieving sub-arcsecond resolution at frequencies below 200 MHz. However, it is technically and logistically challenging to process LOFAR data at this resolution. To date only a handful of publications have exploited this capability. Here we present a calibration strategy that builds on previous high-resolution work with LOFAR. It is implemented in a pipeline using mostly dedicated LOFAR software tools and the same processing framework as the LOFAR Two-metre Sky Survey (LoTSS). We give an overview of the calibration strategy and discuss the special challenges inherent to enacting high-resolution imaging with LOFAR, and describe the pipeline, which is publicly available, in detail. We demonstrate the calibration strategy by using the pipeline on P205+55, a typical LoTSS pointing with an 8 h observation and 13 international stations. We perform in-field delay calibration, solution referencing to other calibrators in the field, self-calibration of these calibrators, and imaging of example directions of interest in the field. We find that for this specific field and these ionospheric conditions, dispersive delay solutions can be transferred between calibrators up to ~1.5° away, while phase solution transferral works well over ~1°. We also demonstrate a check of the astrometry and flux density scale with the in-field delay calibrator source. Imaging in 17 directions, we find the restoring beam is typically ~0.3′′ ×0.2′′ although this varies slightly over the entire 5 deg2field of view. We find we can achieve ~80–300 μJy bm−1image rms noise, which is dependent on the distance from the phase centre; typical values are ~90 μJy bm−1for the 8 h observation with 48 MHz of bandwidth. Seventy percent of processed sources are detected, and from this we estimate that we should be able to image roughly 900 sources per LoTSS pointing. This equates to ~ 3 million sources in the northern sky, which LoTSS will entirely cover in the next several years. Future optimisation of the calibration strategy for efficient post-processing of LoTSS at high resolution makes this estimate a lower limit.