FlexKnot as a Generalised Model of the Sky-averaged 21-cm Signal at z ~ 6-30 in the Presence of Systematics

TL;DR

FlexKnot is a flexible, spline-based model for the global 21-cm signal that outperforms traditional models in reconstructing complex signals amidst systematics, aiding studies of cosmic dawn and reionisation.

Contribution

This paper introduces the FlexKnot function as a novel, adaptable signal model for 21-cm experiments, capable of recovering complex signals and handling systematics better than existing models.

Findings

FlexKnot outperforms Gaussian models in signal reconstruction.

Four to five knots suffice for most realistic signals.

Signals with absorption troughs of 120-450 mK are well recovered.

Abstract

Global 21-cm experiments are built to study the evolution of the Universe between the cosmic dawn and the epoch of reionisation. FlexKnot is a function parameterised by freely moving knots stringed together by splines. Adopting the FlexKnot function as the signal model has the potential to separate the global 21-cm signal from the foregrounds and systematics while being capable of recovering the crucial features given by theoretical predictions. In this paper, we implement the FlexKnot method by integrating twice over a function of freely moving knots interpolated linearly. The function is also constrained at the lower frequencies corresponding to the dark ages by theoretical values. The FlexKnot model is tested in the framework of the realistic data analysis pipeline of the REACH global signal experiment using simulated antenna temperature data. We demonstrate that the FlexKnot model performs better than existing signal models, e.g. the Gaussian signal model, at reconstructing the shape of the true signals present in the simulated REACH data, especially for injected signals with complex structures. The capabilities of the FlexKnot signal model is also tested by introducing various systematics and simulated global signals of different types. These tests show that four to five knots are sufficient to recover the general shape of most realistic injected signals, with or without sinusoidal systematics. We show that true signals whose absorption trough of amplitude between 120 to 450 mK can be well recovered with systematics up to about 50 mK.