Quantifying the non-Gaussianity in the EoR 21-cm signal through bispectrum

TL;DR

This paper uses the bispectrum to quantify and analyze the evolving non-Gaussianity of the EoR 21-cm signal, revealing the contributions of neutral fraction and matter density fluctuations and proposing new detection and visualization methods.

Contribution

It introduces the use of bispectrum sign changes as markers for non-Gaussianity sources and proposes a new visualization approach for signal evolution.

Findings

Bispectrum sign change indicates dominant non-Gaussianity source.

Non-Gaussianity varies with reionization stage and k modes.

Proposed visualization in P(k)-B(k,k,k) space captures signal evolution.

Abstract

The epoch of reionization (EoR) 21-cm signal is expected to be highly non-Gaussian in nature and this non-Gaussianity is also expected to evolve with the progressing state of reionization. Therefore the signal will be correlated between different Fourier modes ($k$). The power spectrum will not be able capture this correlation in the signal. We use a higher-order estimator -- the bispectrum -- to quantify this evolving non-Gaussianity. We study the bispectrum using an ensemble of simulated 21-cm signal and with a large variety of $k$ triangles. We observe two competing sources driving the non-Gaussianity in the signal: fluctuations in the neutral fraction ($x_{\rm HI}$) field and fluctuations in the matter density field. We find that the non-Gaussian contribution from these two sources vary, depending on the stage of reionization and on which $k$ modes are being studied. We show that the sign of the bispectrum works as a unique marker to identify which among these two components is driving the non-Gaussianity. We propose that the sign change in the bispectrum, when plotted as a function of triangle configuration $\cos{\theta}$ and at a certain stage of the EoR can be used as a confirmative test for the detection of the 21-cm signal. We also propose a new consolidated way to visualize the signal evolution (with evolving $\overline{x}_{\rm HI}$ or redshift), through the trajectories of the signal in a power spectrum and equilateral bispectrum i.e. $P(k)-B(k, k, k)$ space.