Impact of Antenna Structure and Orientation on Forward-Modelled Global 21 cm Signal Recovery

Abstract

The redshifted 21 cm absorption trough from cosmic atomic hydrogen is one of the most promising probes of the early Universe, but its detection is challenged by bright foregrounds and instrumental systematics. In this work we quantify the impact of antenna mismodelling on signal recovery within a fully Bayesian, forward-modelled data analysis pipeline. We show that discrepancies between simulated and modelled antenna beams lead to frequency dependent errors in antenna temperature that can bias parameter inference. In particular, we demonstrate that orientation mismatches at the level of 0.25 degrees can significantly bias recovered signal parameters in typical observing scenarios. However, we also show that Bayesian evidence can be used to infer antenna orientation within this precision by scanning over model realisations. For structural mismodelling, we find that broadband recovery of all signal parameters requires accurate beam knowledge, but that partial recovery remains possible. Signal frequency and width can be robustly recovered under restricted frequency bands even when the antenna structure is imperfectly modelled, but signal depth is highly sensitive to beam errors. These results quantify the level of beam knowledge required for forward-modelled global 21 cm experiments and highlight the importance of observing strategy and antenna design in mitigating beam-sky coupling systematics.

Figures (9)

• Figure 1: Side profile of the REACH radiometer taken from the north west side of the instrument. The image shows the REACH serrated ground plane elevated 1 m above the soil by a series of wooden posts. On top of the metal ground plane in the centre of the image we see a table, painted white, supporting the REACH hexagonal dipole blades. Image courtesy of Rohan Patel.
• Figure 2: Measuring device used to determine the topography of the ground plane of the REACH instrument. Location of implement is marked with yellow, non-conductive twine, and numbered.
• Figure 3: Topographic map of the ground plane of the REACH antenna. The black dotted lines indicate the dimensions of the ground plane as designed, the solid green lines the measured dimensions of the extent of the ground plane. The black crosses indicate measurement locations on the ground plane, the grey crosses represent measurements which were outside the ground plane bounds following fitting and discounted. The table upon which the dipole sits is denoted in purple. True north is located along the y-axis, magnetic north is indicated by the red arrow offset by $24^\circ$.
• Figure 4: Percentage change in the directivity of the REACH dipole antenna as complexity of the modelled antenna increases. Each panel shows a waterfall plot of antenna chromaticity. The 'cubehelix' Green2011AImages colourmap giving the mean percentage directivity difference per unit solid angle, for the two left-hand columns this is for a given azimuthal angle, averaged across all zenith angle, and for the two right-handed columns this shows directivity at a given angle from zenith for all values of azimuthal angle. The left hand column shows the cumulative change in directivity from an idealised antenna model (FIS) as the complexity of the modelled antenna increases, and the right hand column indicates the incremental percentage change in directivity with each layer of additional complexity. The ideal antenna has a ground plane built to originally specified dimensions, with a dipole angled such that the y axis are is aligned to the north-south axis, and the ground plane is perfectly flat. The top row changes the ground plane structure from having the idealised dimensions with perfect serrations, compared to those measured on site (FRS). The second row rotates the dipole relative to the ground plane to match measured values (FRA). The bottom row adapts the topography of the ground plane to match that measured on site (BRA).
• Figure 5: Absolute and fractional difference of antenna temperature per unit frequency when there is an angular mismatch between the antenna observing the sky in the simulation and model. Each coloured line refers to a different angular mismatch in degrees, as shown by the legend. The gain of the antenna beam is discretely sampled at 1 MHz intervals. The dotted black line and grey shaded region indicates a fractional difference of 100 parts per million. Each set of data represents the average antenna temperature over the course of three hours. The left hand side plots represent a 'Galaxy Down' case in which our simulation begins 2019-10-01 00:00:00, during this period the galactic centre is near or below the horizon. The right hand side plots represent a 'Galaxy Up' case in which our simulation begins 2019-07-01 00:00:00, during this period the galactic centre is above the horizon.