Understanding the HERA Phase I receiver system with simulations and its impact on the detectability of the EoR delay power spectrum

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. Thus, this new study shows that the detection of the EoR signal with HERA Phase I will be more challenging than expected. On the other hand, it improves our understanding of the telescope, which is essential to mitigate the instrument chromaticity.

Effects of model incompleteness on the drift-scan calibration of radio telescopes

“…Kern et al 2019Kern et al , 2020a for HERA Phase-I calibration for 21-cm analyses). The HERA dipole-feed was given detailed studies with simulations by Fagnoni et al (2021), who produced the beam model that we use here. 1 As a comparison point, we also include an isolated dipole antenna in our analysis.…”

“…Methods to estimate primary beams include electromagnetic simulations of radio antennas using software packages e.g. or (Fagnoni et al 2021;Mahesh et al 2021), holographic beam mapping techniques (Berger et al 2016;Iheanetu et al 2019;Asad et al 2021), beam mapping using bright radio sources transiting through the sky (Baars et al 1977;Nunhokee et al 2020) as well as artificial sources e.g. satellites (Neben et al 2016;Line et al 2018), and more recently using artificial radio sources mounted on Unmanned Aerial Vehicles (e.g.…”

“…We further down-select auto-correlations for 8 antennas and single polarization (XX) from the observation data. To generate the sky model beam averaged sky temperature model for calibration, we use the GSM2008 sky model and the HERA dipole feed primary beam model from Fagnoni et al (2021). We also account for the CMB temperature and the ground pickup (assuming a constant ground temperature of 300 K) in the calculation of the sky temperature model to obtain a more realistic calibration model.…”

“…Figure 12 provides a comparison of gain amplitude |𝑔| = √︁ Ĝ obtained using the method presented here with the gain amplitude obtained by calibrating cross-correlation visibilities as described in Kern et al (2020a), and a comparison of Trxr obtained with expected value of 𝑇 rxr ∼ 170 K for the HERA dish with dipole feed (Fagnoni et al 2021). We observe that the average gain amplitude obtained from auto-correlations matches very well with the frequency smoothed gain amplitude obtained from cross-correlation visibilities of the same dataset.…”

Simulations of primary beam effects on the cosmic bispectrum phase observed with the Hydrogen Epoch of Reionization Array

“…Closure phase quantities, however, are affected by directiondependent effects such as varying antenna primary beams due to mutual coupling induced by a very compact configuration (e.g., Fagnoni et al 2021;Josaitis et al 2021). Variations of primary beams across the array invalidates the assumption of redundancy (i.e., that baselines with the same length and orientation measure exactly the same signal from the sky) which is the core of the HERA calibration strategy (Dillon et al 2020).…”