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The three-point correlation function (3PCF) of the 21cm brightnesstemperature from the Epoch of Reionization (EoR) probes complementaryinformation to the commonly studied two-point correlation function (2PCF) aboutthe morphology of ionized regions. We investigate the 21cm 2PCF and 3PCF inconfiguration space using semi-numerical simulations and test whether they canbe described by the local quadratic bias model. We find that fits of bias modelpredictions for the 2PCF and 3PCF deviate from our measurements by $\sim 20\%$at scales above the typical size of ionized regions ($\simeq 30$ Mpc) and atearly times with global neutral fractions of $\langle x_{\rm HI} \rangle\gtrsim 0.7$. At later times and smaller scales these deviations increasestrongly, indicating a break down of the bias model. The 2PCF and 3PCF fits ofthe linear bias parameter agree at the $10\%$ level for different EoR modelconfigurations. This agreement holds, when adding redshift space distortions tothe simulations. The relation between spatial fluctuations in the matterdensity and the 21cm signal, as predicted by the bias model, is consistent withdirect measurements of this relation in simulations for large smoothing scales($\gtrsim 30$ Mpc). From this latter test we conclude that negative amplitudesof the 21cm 3PCF result from negative bias parameters, which describe theanti-correlation between the matter over-densities and the 21cm signal duringthe EoR. However, a more detailed interpretation of the bias parameters mayrequire a description of non-local contributions to the bias model.

Signatures of Cosmic Reionization on the 21cm 2- and 3-point Correlation Function I: Quadratic Bias Modeling