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