On the spin-temperature evolution during the epoch of reionization

Simulations estimating the brightness temperature (δT_b) of the redshifted 21 cm from the epoch of reionization (EoR) often assume that the spin temperature (T_s) is decoupled from the background cosmic microwave background (CMB) temperature and is much larger than it, i.e. T_s T_CMB. Although a valid assumption towards the later stages of the reionization process, it does not necessarily hold at the earlier epochs. Violation of this assumption will lead to fluctuations in δT_b that are driven neither by density fluctuations nor by H ii regions. Therefore, it is vital to calculate the spin temperature self-consistently by treating the Lyα and collisional coupling of T_s to the kinetic temperature, T_k. In this paper we develop an extension to the bears algorithm, originally developed to model reionization history, to include these coupling effects. Here, we simulate the effect in ionization and heating for three models in which the reionization is driven by stars, mini-QSOs or a mixture of both. We also perform a number of statistical tests to quantify the imprint of the self-consistent inclusion of the spin-temperature decoupling from the CMB. We find that the evolution of the spin temperature has an impact on the measured signal especially at redshifts higher than 10 and such evolution should be taken into account when one attempts to interpret the observational data.