Making robust reionisation 21 cm signal predictions with C21LRT

The redshifted 21 cm line signal is a unique probe for the Epoch of Reionisation by tracking the evolution of the averaged intergalactic medium properties as well as their fluctuations, enabling tomographic studies. Accurate modelling of the 21 cm global and power spectrum signals is crucial for interpreting measurements from current and forthcoming 21 cm experiments. Theoretical predictions usually post-process reionisation simulations with optical depth approximation, which treats local line broadening and peculiar velocities approximately and often leads to divergences due to its velocity gradient term. I will present our cosmological 21 cm line radiative transfer (C21LRT) formulation which resolve these issues naturally. I will compare the 21-cm signals calculated with C21LRT and the conventional optical depth method, and highlight the need to fully quantify the uncertainties of using the optical depth method for exploring the 21-cm signals associated from various possible reionisation scenarios.