The HI 21cm absorption line is masked by bright foregrounds and systematic distortions that arise due to the chromaticity of the antenna used to make the observation coupling to the spectral inhomogeneity of these foregrounds. We demonstrate that these distortions are sufficient to conceal the 21cm signal when the antenna is not perfectly achromatic and that simple corrections assuming a constant spatial distribution of foreground power are insufficient to overcome them. We then propose a new physics-motivated method of modelling the foregrounds of 21cm experiments in order to fit the chromatic distortions as part of the foregrounds. This is done by generating a simulated sky model across the observing band by dividing the sky into $N$ regions and scaling a base map assuming a distinct uniform spectral index in each region. The resulting sky map can then be convolved with a model of the antenna beam to give a model of foregrounds and chromaticity parameterised by the spectral indices of the $N$ regions. We demonstrate that fitting this model for varying $N$ using a Bayesian nested sampling algorithm and comparing the results using the evidence allows the 21cm signal to be reliably detected in data of a relatively smooth conical log spiral antenna. We also test a much more chromatic conical sinuous antenna and find this model will not produce a reliable signal detection, but in a manner that is easily distinguishable from a true detection.

A general Bayesian framework for foreground modelling and chromaticity correction for global 21 cm experiments

Observations of the 21-cm line from primordial hydrogen promise to be one of the best tools to study the early epochs of the Universe: the dark ages, the cosmic dawn and the subsequent epoch of reionization. In 2018, the Experiment to Detect the Global Epoch of Reionization Signature (EDGES) caught the attention of the cosmology community with a potential detection of an absorption feature in the sky-averaged radio spectrum centred at 78 MHz. The feature is deeper than expected, and, if confirmed, would call for new physics. However, different groups have re-analysed the EDGES data and questioned the reliability of the signal. The Radio Experiment for the Analysis of Cosmic Hydrogen (REACH) is a sky-averaged 21-cm experiment aiming at improving the current observations by tackling the issues faced by current instruments related to residual systematic signals in the data. The novel experimental approach focuses on detecting and jointly explaining these systematics together with the foregrounds and the cosmological signal using Bayesian statistics. To achieve this, REACH features simultaneous observations with two different antennas, an ultra-wideband system (redshift range about 7.5 to 28) and a receiver calibrator based on in-field measurements. Simulated observations forecast percent-level constraints on astrophysical parameters, potentially opening up a new window to the infant Universe. The Radio Experiment for the Analysis of Cosmic Hydrogen (REACH) aims to detect the sky-averaged 21-cm neutral hydrogen line from the early Universe by jointly analysing the cosmological signal, foreground emission and systematic effects. 

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The REACH radiometer for detecting the 21-cm hydrogen signal from redshift z ≈ 7.5–28

An ultra-long-wavelength radio telescope on the far side of the Moon has significant advantages compared to Earth-based and Earth-orbiting telescopes, including: 1. Enabling observations of the Universe at wavelengths longer than 10 meters (i.e., frequencies below 30 MHz), wavelengths at which critical cosmological or extrasolar planetary signatures are predicted to appear, yet cannot be observed from the ground due to absorption from the Earth's ionosphere; and 2. The Moon acts as a physical shield that isolates a far-side lunar-surface telescope from radio interference from sources on the Earth's surface, the ionosphere, Earth-orbiting satellites, and the Sun's radio emission during the lunar night. In this paper, we present the conceptual design of the Lunar Crater Radio Telescope (LCRT) on the far side of the Moon. We propose to deploy a wire mesh using wall-climbing DuAxel robots in a 3–5 km diameter crater, with a suitable depth-to-diameter ratio, to form a parabolic reflector with a 1 km diameter. LCRT will be the largest filled-aperture radio telescope in the Solar System; larger than the former Arecibo telescope (305 m diameter, 3 cm - 1 m wavelength band, 0.3-10 GHz frequency band) and the Five-hundred-meter Aperture Spherical radio Telescope (FAST) (500 m diameter, 0.1-4.3 m wavelength band, 60–3000 MHz frequency band). LCRT's science objective is to track the evolution of the neutral intergalactic medium before and during the formation of the first stars in the 10–100 m wavelength band (3–30 MHz frequency band), which is consistent with priorities identified in the Astrophysics decadal survey. We describe LCRT's science objectives and the key technology challenges that need to be overcome to make this concept a reality. We envisage that LCRT will open a new window for humanity's exploration of the Universe.

Conceptual Design of the Lunar Crater Radio Telescope (LCRT) on the Far Side of the Moon

Global 21cm experiments aim to measure the sky averaged HI absorption signal from cosmic dawn and the epoch of reionisation. However, antenna chromaticity coupling to bright foregrounds can introduce distortions into the observational data of such experiments. We demonstrate a method for guiding the antenna design of a global experiment through data analysis simulations. This is done by performing simulated observations for a range of inserted 21cm signals, then attempting to identify the signals with a data analysis pipeline. We demonstrate this method on five antennae that were considered as potential designs for the Radio Experiment for the Analysis of Cosmic Hydrogen (REACH); a conical log spiral antenna, an inverted conical sinuous antenna and polygonal-, square- and elliptical-bladed dipoles. We find that the log spiral performs significantly better than the other antennae tested, able to correctly and confidently identify every inserted 21cm signal. In second place is the polygonal dipole antenna, which was only unable to detect signals with both very low amplitudes of 0.05K and low centre frequency of 80MHz. The conical sinuous antenna was found to perform least accurately, only able to detect the highest amplitude 21cm signals, and even then with biases. We also demonstrate that, due to the non-trivial nature of chromatic distortion and the processes of correcting for it, these are not the results that could have been expected superficially from the extent of chromatic variation in each antenna.

Informing antenna design for sky-averaged 21-cm experiments using a simulated Bayesian data analysis pipeline

We use EDGES measurements to determine scale and zero-level corrections to the diffuse radio surveys by Guzm\'an et al. at $45$ MHz and Landecker & Wielebinski at $150$ MHz. We find that the Guzm\'an et al. map requires a scale correction of $1.076 \pm 0.034$ ($2\sigma$) and a zero-level correction of $-160 \pm 78$ K ($2\sigma$) to best-fit the EDGES data. For the Landecker & Wielebinski map, the scale correction is $1.112 \pm 0.023$ ($2\sigma$) and the zero-level correction is $0.7 \pm 6.0$ K ($2\sigma$). The correction uncertainties are dominated by systematic effects, of which the most significant are uncertainty in the calibration of the EDGES receivers, antenna pointing, and tropospheric and ionospheric effects. We propagate the correction uncertainties to estimate the uncertainties in the corrected maps themselves and find that the $2\sigma$ uncertainty in the map brightness temperature is in the range $3.2-7.5\%$ for the Guzm\'an et al. map and $2.1-9.0\%$ for the Landecker & Wielebinski map, with the largest percent uncertainties occurring at high Galactic latitudes. The corrected maps could be used to improve existing diffuse low-frequency radio sky models, which are essential tools in analyses of cosmological $21$ cm observations, as well as to investigate the existence of a radio monopole excess above the cosmic microwave background and known Galactic and extragalactic contributions.

Absolute Calibration of Diffuse Radio Surveys at 45 and 150 MHz

We modelled the two major layer of Earth's ionosphere, the F-layer and the D-layer, by a simplified spatial model with temporal variance to study the chromatic ionospheric effects on global 21-cm observations. From the analyses, we found that the magnitude of the ionospheric disruptions due to ionospheric refraction and absorption can be greater than the expected global 21-cm signal, and the variation of its magnitude can differ, depending on the ionospheric conditions. Within the parameter space adopted in the model, the shape of the global 21-cm signal is distorted after propagating through the ionosphere, while its amplitude is weakened. It is observed that the ionospheric effects do not cancel out over time, and thus should be accounted for in the foreground calibration at each timestep to account for the chromaticity introduced by the ionosphere.

Emma Shen

Papers

Quantifying ionospheric effects on global 21-cm observations