低频射电干涉观测中基于能见度的角功率谱估计
Visibility based angular power spectrum estimation in low frequency radio interferometric observations
摘要：
我们提出了两个估计器来量化的天空信号的角功率谱直接从无线电干涉观测测量的vibrations。这与前景和埋在其中的宇宙学21厘米信号都有关。这里的讨论仅限于银河系同步辐射，最主要的前景成分后，点源删除。我们的理论分析验证使用模拟在150 MHz，主要是GMRT，也简要地为LOFAR。“裸估计量”使用测量的活力度的成对相关性，而“渐变网格估计量”使用在uv平面中网格化后的活力度。前者非常精确，但对于大数据来说计算成本很高。后者具有较低的精度，但需要较少的计算时间，这是成比例的数据量。后者还允许天空响应逐渐变细，从而导致旁瓣抑制，这是前景去除的有用成分。这两种估计器都避免了由于系统噪声而产生的正偏差。我们考虑增益的幅度和相位误差，以及作为可能的误差源的w项。我们发现，估计的角功率谱是指数敏感的相位误差的方差，但幅度误差不敏感。估计器的统计不确定性受幅相误差的影响。w项在我们感兴趣的角度尺度上没有显著影响。我们提出了一个有效的工具来观测量化前景和宇宙学的21厘米信号的锥形网格估计。
英文摘要：
We present two estimators to quantify the angular power spectrum of the sky signal directly from the visibilities measured in radio interferometric observations. This is relevant for both the foregrounds and the cosmological 21-cm signal buried therein. The discussion here is restricted to the Galactic synchrotron radiation, the most dominant foreground component after point source removal. Our theoretical analysis is validated using simulations at 150 MHz, mainly for GMRT and also briefly for LOFAR. The Bare Estimator uses pairwise correlations of the measured visibilities, while the Tapered Gridded Estimator uses the visibilities after gridding in the uv plane. The former is very precise, but computationally expensive for large data. The latter has a lower precision, but takes less computation time which is proportional to the data volume. The latter also allows tapering of the sky response leading to sidelobe suppression, an useful ingredient for foreground removal. Both estimators avoid the positive bias that arises due to the system noise. We consider amplitude and phase errors of the gain, and the w-term as possible sources of errors . We find that the estimated angular power spectrum is exponentially sensitive to the variance of the phase errors but insensitive to amplitude errors. The statistical uncertainties of the estimators are affected by both amplitude and phase errors. The w-term does not have a significant effect at the angular scales of our interest. We propose the Tapered Gridded Estimator as an effective tool to observationally quantify both foregrounds and the cosmological 21-cm signal.
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