Probing non-Gaussian features in the HI distribution at the epoch of reionization

The HI distribution at the epoch of reionization (EOR) is largely determined by the sizes and distribution of the ionized regions. In the scenario where the ionized regions have comoving sizes of the order of a few Mpc, the large scale statistical properties of the HI distribution are dominated by the Poisson noise of the discrete ionized regions, and it is highly non-Gaussian. We investigate the possibility of probing reionization by studying these non-Gaussian features using future radio interferometric observations of redshifted 21 cm HI radiation. We develop a formalism relating correlations between the visibilities measured at three different baselines and frequencies to the bispectrum of HI fluctuations. For visibilities at the same frequency, this signal is found to be of the same order as the two visibility correlation which probes the HI power spectrum. For visibilities at different frequencies, we find that the correlations decay within a frequency difference of ~1 MHz. This implies that it is, in principle, straightforward to extract this HI signal from various contaminants which are believed to have a continuum spectra and are expected to be correlated even at large frequency separations.