The Epoch of Reionization 21-cm Bispectrum: The impact of light-cone
  effects and detectability
  We study the spherically averaged bispectrum of the 21-cm signal from the
Epoch of Reionization (EoR). This metric provides a quantitative measurement of
the level of non-Gaussianity of the signal which is expected to be high. We
focus on the impact of the light-cone effect on the bispectrum and its
detectability with the future SKA-Low telescope. Our investigation is based on
a single reionization light-cone model and an ensemble of 50 realisations of
the 21-cm signal to estimate the cosmic variance errors. We calculate the
bispectrum with a new, optimised direct estimation method, DviSukta which
calculates the bispectrum for all possible unique triangles. We find that the
light-cone effect becomes important on scales $k_1 \lesssim 0.1\,{\rm
Mpc}^{-1}$ where for most triangle shapes the cosmic variance errors dominate.
Only for the squeezed limit triangles, the impact of the light-cone effect
exceeds the cosmic variance. Combining the effects of system noise and cosmic
variance we find that $\sim 3\sigma$ detection of the bispectrum is possible
for all unique triangle shapes around a scale of $k_1 \sim 0.2\,{\rm
Mpc}^{-1}$, and cosmic variance errors dominate above and noise errors below
this length scale. Only the squeezed limit triangles are able to achieve a more
than $5\sigma$ significance over a wide range of scales, $k_1 \lesssim
0.8\,{\rm Mpc}^{-1}$. Our results suggest that among all the possible triangle
combinations for the bispectrum, the squeezed limit one will be the most
measurable and hence useful.
