Quantifying suppression of the cosmological 21-cm signal due to direction-dependent gain calibration in radio interferometers
Quantifying suppression of the cosmological 21-cm signal due to direction-dependent gain calibration in radio interferometers
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DOI：
10.1093/mnras/sty3444
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发表时间：
2018-09
影响因子：
4.8
通讯作者：
A. M. Sardarabadi;L. Koopmans
中科院分区：
文献类型：
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作者：
A. M. Sardarabadi;L. Koopmans
中性氢的21厘米信号有望成为研究宇宙黎明和再电离时代的重要信息来源。然而，如果不充分减轻数据中的强污染信号，则难以检测到它，需要精确的仪器增益校准。为了检测21 cm信号，分析得到的校准后残差数据。然而，驻留在这些残差中的21厘米信号会受到方向相关校准（DDC）与天空模型去除相结合的过程的强烈影响。这一影响，特别是对21厘米信号的频率特性的影响，仍然很难量化。在本文中，我们推导出一个新的数学形式主义如何计算的21厘米信号的抑制DDC和天空模型去除。我们还展示了如何先验信息的频率行为的仪器，如频谱平滑度，可以用来减少信号抑制。使用基于LOFAR-EoR校准设置的现实模拟来测试理论结果。结果表明，如果仪器增益在几个MHz上本质上是平滑的（例如，在类似于10 MHz的带宽上的多项式），这足以允许DDC在多达100个方向上具有有限的和可量化的21 cm信号功率谱的抑制。我们还表明，在校准过程中更不完整的天空模型导致更大的21厘米信号抑制，即使仪器增益模型是完全平滑的。这一结果对未来具有不同站波束的射电望远镜有直接的影响，其中DDC可能是必要的（例如SKA-Low）。
The 21-cm signal of neutral hydrogen promises to be an important source of information for the study of the cosmic dawn and epoch of reionization. However, its detection is difficult without sufficient mitigation of strong contaminating signals in the data, requiring accurate instrument gain calibration. To detect the 21-cm signal, one analyses the resulting post-calibration residual data. The 21-cm signal residing in these residuals, however, can be strongly affected by the process of direction-dependent calibration (DDC) combined with sky-model removal. The impact of this, in particular on the frequency behaviour of the 21-cm signal, has remained poorly quantified. In this paper, we derive a new mathematical formalism on how to calculate the suppression of the 21-cm signal during DDC and sky-model removal. We also show how a priori information about the frequency behaviour of the instrument, such as spectral smoothness, can be utilized to reduce signal suppression. The theoretical results are tested using a realistic simulation based on the LOFAR-EoR calibration set-up. The results show that if the instrumental gains are intrinsically smooth over several MHz (e.g. polynomials over a bandwidth of similar to 10 MHz), this is sufficient to allow for DDC in as many as similar to 100 directions with limited and quantifiable suppression of the 21-cm signal power spectrum. We also demonstrate that more incomplete sky models during calibration lead to larger 21-cm signal suppression, even if the instrumental gain models are fully smooth. This result has immediate consequences for future radio telescopes with non-identical station beams, where DDC might be necessary (e.g. SKA-Low).