Redshifted 21-cm bispectrum II: Impact of the spin temperature
  fluctuations and redshift space distortions on the signal from the Cosmic
  Dawn
  We present a study of the 21-cm signal bispectrum (which quantifies the
non-Gaussianity in the signal) from the Cosmic Dawn (CD). For our analysis, we
have simulated the 21-cm signal using radiative transfer code GRIZZLY, while
considering two types of sources (mini-QSOs and HMXBs) for Ly$\alpha$ coupling
and the X-ray heating of the IGM. Using this simulated signal, we have, for the
first time, estimated the CD 21-cm bispectra for all unique $k$-triangles and
for a range of $k$ modes. We observe that the redshift evolution of the
bispectra magnitude and sign follow a generic trend for both source models.
However, the redshifts at which the bispectra magnitude reach their maximum and
minimum values and show their sign reversal depends on the source model. When
the Ly$\alpha$ coupling and the X-ray heating of the IGM occur simultaneously,
we observe two consecutive sign reversals in the bispectra for small
$k$-triangles (irrespective of the source models). One arising at the beginning
of the IGM heating and the other at the end of Ly$\alpha$ coupling saturation.
This feature can be used in principle to constrain the CD history and/or to
identify the specific CD scenarios. We also quantify the impact of the spin
temperature ($T_{\rm S}$) fluctuations on the bispectra. We find that $T_{\rm
S}$ fluctuations have maximum impact on the bispectra magnitude for small
$k$-triangles and at the stage when Ly$\alpha$ coupling reaches saturation.
Furthermore, we are also the first to quantify the impact of redshift space
distortions (RSD), on the CD bispectra. We find that the impact of RSD on the
CD 21-cm bispectra is significant ($> 20\%$) and the level depends on the
stages of the CD and the $k$-triangles for which the bispectra are being
estimated.
