On the spin-temperature evolution during the epoch of reionization
  Simulations estimating the differential brightness temperature of the
redshifted 21-cm from the epoch of reionization (EoR) often assume that the
spin temperature is decoupled from the background CMB temperature and is much
larger than it. Although a valid assumption towards the latter stages of the
reionization process, it does not necessarily hold at the earlier epochs.
Violation of this assumption will lead to fluctuations in differential
brightness temperature that are neither driven by density fluctuations nor by
HII regions. Therefore, it is vital to calculate the spin temperature
self-consistently by treating the Lyman-alpha and collisional coupling of spin
temperature to the kinetic temperature. 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, miniqsos
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 specially at redshifts higher than 10 and such evolution
should be taken into account when one attempts to interpret the observational
data.
