Antenna beam characterization for the global 21-cm experiment LEDA and its impact on signal model parameter reconstruction
ABSTRACT

Cosmic dawn, the onset of star formation in the early universe, can in principle be studied via the 21-cm transition of neutral hydrogen, for which a sky-averaged absorption signal, redshifted to MHz frequencies, is predicted to be O(10–100) mK. Detection requires separation of the 21-cm signal from bright chromatic foreground emission due to Galactic structure, and the characterization of how it couples to instrumental response. In this work, we present characterization of antenna gain patterns for the Large-aperture Experiment to detect the Dark Ages (LEDA) via simulations, assessing the effects of the antenna ground-plane geometries used, and measured soil properties. We then investigate the impact of beam pattern uncertainties on the reconstruction of a Gaussian absorption feature. Assuming the pattern is known and correcting for the chromaticity of the instrument, the foregrounds can be modelled with a log-polynomial, and the 21-cm signal identified with high accuracy. However, uncertainties on the soil properties lead to percentage changes in the chromaticity that can bias the signal recovery. The bias can be up to a factor of two in amplitude and up to few  per cent in the frequency location. These effects do not appear to be mitigated by larger ground planes, conversely gain more »

Authors:
; ; ; ; ; ;
Publication Date:
NSF-PAR ID:
10374690
Journal Name:
Monthly Notices of the Royal Astronomical Society
Volume:
515
Issue:
2
Page Range or eLocation-ID:
p. 1580-1597
ISSN:
0035-8711
Publisher:
Oxford University Press
2. Abstract The detection of the Epoch of Reionization (EoR) delay power spectrum using a ”foreground avoidance method” highly depends on the instrument chromaticity. The systematic effects induced by the radio-telescope spread the foreground signal in the delay domain, which contaminates the EoR window theoretically observable. Applied to the Hydrogen Epoch of Reionization Array (HERA), this paper combines detailed electromagnetic and electrical simulations in order to model the chromatic effects of the instrument, and quantify its frequency and time responses. In particular, the effects of the analogue receiver, transmission cables, and mutual coupling are included. These simulations are able to accurately predict the intensity of the reflections occurring in the 150-m cable which links the antenna to the back-end. They also show that electromagnetic waves can propagate from one dish to another one through large sections of the array due to mutual coupling. The simulated system time response is attenuated by a factor 104 after a characteristic delay which depends on the size of the array and on the antenna position. Ultimately, the system response is attenuated by a factor 105 after 1400 ns because of the reflections in the cable, which corresponds to characterizable k∥-modes above 0.7 $h\,\,\rm {Mpc}^{-1}$ at 150 MHz.more »
5. ABSTRACT Future generations of radio interferometers targeting the 21 cm signal at cosmological distances with N ≫ 1000 antennas could face a significant computational challenge in building correlators with the traditional architecture, whose computational resource requirement scales as $\mathcal {O}(N^2)$ with array size. The fundamental output of such correlators is the cross-correlation products of all antenna pairs in the array. The FFT-correlator architecture reduces the computational resources scaling to $\mathcal {O}(N\log {N})$ by computing cross-correlation products through a spatial Fourier transform. However, the output of the FFT-correlator is meaningful only when the input antenna voltages are gain- and phase-calibrated. Traditionally, interferometric calibration has used the $\mathcal {O}(N^2)$ cross-correlations produced by a standard correlator. This paper proposes two real-time calibration schemes that could work in parallel with an FFT-correlator as a self-contained $\mathcal {O}(N\log {N})$ correlator system that can be scaled to large-N redundant arrays. We compare the performance and scalability of these two calibration schemes and find that they result in antenna gains whose variance decreases as 1/log N with increase in the size of the array.