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We present a study of the 21-cm signal bispectrum (which quantifies thenon-Gaussianity in the signal) from the Cosmic Dawn (CD). For our analysis, wehave simulated the 21-cm signal using radiative transfer code GRIZZLY, whileconsidering two types of sources (mini-QSOs and HMXBs) for Ly$\alpha$ couplingand the X-ray heating of the IGM. Using this simulated signal, we have, for thefirst time, estimated the CD 21-cm bispectra for all unique $k$-triangles andfor a range of $k$ modes. We observe that the redshift evolution of thebispectra magnitude and sign follow a generic trend for both source models.However, the redshifts at which the bispectra magnitude reach their maximum andminimum values and show their sign reversal depends on the source model. Whenthe 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 beginningof 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 toidentify the specific CD scenarios. We also quantify the impact of the spintemperature ($T_{\rm S}$) fluctuations on the bispectra. We find that $T_{\rmS}$ 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 spacedistortions (RSD), on the CD bispectra. We find that the impact of RSD on theCD 21-cm bispectra is significant ($> 20\%$) and the level depends on thestages of the CD and the $k$-triangles for which the bispectra are beingestimated.

Redshifted 21-cm bispectrum II: Impact of the spin temperature fluctuations and redshift space distortions on the signal from the Cosmic Dawn