SARAS 2: a spectral radiometer for probing cosmic dawn and the epoch of reionization through detection of the global 21-cm signal
SARAS 2: a spectral radiometer for probing cosmic dawn and the epoch of reionization through detection of the global 21-cm signal
复制标题
SARAS 2：光谱辐射计，通过检测全球 21 厘米信号来探测宇宙黎明和再电离时代
DOI：
10.1007/s10686-018-9584-3
复制
发表时间：
2018
影响因子：
3
通讯作者：
K. Srivani
中科院分区：
文献类型：
--
作者：
Saurabh Singh;R. Subrahmanyan;N. Shankar;M. S. Rao;B. Girish;A. Raghunathan;R. Somashekar;K. Srivani
来自宇宙黎明（CD）和再电离时代（EoR）的全球21厘米信号，在红移上，探测了第一辐射源的性质以及银河系间介质（IGM）的物理学。由于该信号被预测为极其微弱，具有宽的分数带宽，并且位于由银河系和银河系外前景以及射频干扰主导的频率范围内，因此信号的检测是一项艰巨的任务。实验的关键是通过仪器表示天空信号的方式。设计一个能够很好地校准频谱带通和附加杂散信号的系统，并且任何校准残差都不会模仿信号，这是至关重要的。背景无线电频谱的成形天线测量（SARAS）是一项正在进行的实验，旨在检测全球21厘米信号。在这里，我们提出的SARAS 2系统的设计理念，并讨论其性能和局限性的基础上，实验室和现场测量。用各种终端（包括天线阻抗的网络模型）替换天线的实验室测试表明，增益校准和内部加性信号的建模不会留下傅立叶幅度超过2 mK的残差，也不会留下幅度超过2 mK的25 MHz宽度的残差高斯。因此，即使考虑到天线的反射和辐射效率损失，SARAS 2系统也能够检测到复杂的21厘米剖面，其水平与目前流行的热重子演化模型预测的水平相同。
The global 21-cm signal from Cosmic Dawn (CD) and the Epoch of Reionization (EoR), at redshifts, probes the nature of first sources of radiation as well as physics of the Inter-Galactic Medium (IGM). Given that the signal is predicted to be extremely weak, of wide fractional bandwidth, and lies in a frequency range that is dominated by Galactic and Extragalactic foregrounds as well as Radio Frequency Interference, detection of the signal is a daunting task. Critical to the experiment is the manner in which the sky signal is represented through the instrument. It is of utmost importance to design a system whose spectral bandpass and additive spurious signals can be well calibrated and any calibration residual does not mimic the signal. Shaped Antenna measurement of the background RAdio Spectrum (SARAS) is an ongoing experiment that aims to detect the global 21-cm signal. Here we present the design philosophy of the SARAS 2 system and discuss its performance and limitations based on laboratory and field measurements. Laboratory tests with the antenna replaced with a variety of terminations, including a network model for the antenna impedance, show that the gain calibration and modeling of internal additive signals leave no residuals with Fourier amplitudes exceeding 2 mK, or residual Gaussians of 25 MHz width with amplitudes exceeding 2 mK. Thus, even accounting for reflection and radiation efficiency losses in the antenna, the SARAS 2 system is capable of detection of complex 21-cm profiles at the level predicted by currently favoured models for thermal baryon evolution.