National Radio Astronomy Observatory, Socorro, New Mexico 87801, USA.
Arizona State University, School of Earth and Space Exploration, Tempe, Arizona 85287, USA.
Phys Rev Lett. 2018 Jun 22;120(25):251301. doi: 10.1103/PhysRevLett.120.251301.
Detecting neutral hydrogen (H i) via the 21 cm line emission from the intergalactic medium at z≳6 has been identified as one of the most promising probes of the epoch of cosmic reionization-a major phase transition of the Universe. However, these studies face severe challenges imposed by the bright foreground emission from cosmic objects. Current techniques require precise instrumental calibration to separate the weak H i line signal from the foreground continuum emission. We propose to mitigate this calibration requirement by using measurements of the interferometric bispectrum phase. The bispectrum phase is unaffected by antenna-based direction-independent calibration errors and hence for a compact array it depends on the sky brightness distribution only (subject to the usual thermal-like noise). We show that the bispectrum phase of the foreground synchrotron continuum has a characteristically smooth spectrum relative to the cosmological line signal. The two can be separated effectively by exploiting this spectral difference using Fourier techniques, while eliminating the need for precise antenna-based calibration of phases introduced by the instrument, and the ionosphere, inherent in existing approaches. Using fiducial models for continuum foregrounds, and for the cosmological H i signal, we show the latter should be detectable in bispectrum phase spectra, with reasonable significance at |k_{∥}|≳0.5h Mpc^{-1}, using existing instruments. Our approach will also benefit other H i intensity mapping experiments that face similar challenges, such as those measuring baryon acoustic oscillations (BAO).
通过检测红移 z≳6 处的星系际介质中的中性氢（H i）的 21 厘米谱线辐射，已经确定其是宇宙再电离时期——宇宙主要相变之一的最有前途的探测手段之一。然而，这些研究面临着来自宇宙物体的强前景辐射带来的严峻挑战。目前的技术需要精确的仪器校准来将微弱的 H i 谱线信号与前景连续谱辐射分离。我们提议通过使用干涉测量的双谱相位测量来减轻这种校准要求。双谱相位不受基于天线的与方向无关的校准误差的影响，因此对于紧凑阵列，它仅取决于天空亮度分布（受通常的类热噪声的影响）。我们表明，前景同步加速器连续谱的双谱相位相对于宇宙学谱线信号具有特征平滑的谱。可以通过利用傅里叶技术来利用这种谱差异有效地将两者分离，同时消除了对仪器引入的相位以及固有电离层进行精确基于天线的校准的需要，而这些是现有方法所必需的。使用连续谱前景的基准模型，以及宇宙学 H i 信号的基准模型，我们表明，使用现有仪器，在后一种情况下，双谱相位谱中应该可以检测到后者，在 |k_{∥}|≳0.5h Mpc^{-1} 处具有合理的显著性。我们的方法也将使其他面临类似挑战的 H i 强度测绘实验受益，例如那些测量重子声学振荡（BAO）的实验。
