  With a state-of-the-art numerical method for solving the
integral-differential equation of radiative transfer, we investigate the flux
of the Ly$\alpha$ photon $\nu_0$ emergent from an optically thick halo
containing a central light source. Our focus is on the time-dependent effects
of the resonant scattering. We first show that the frequency distribution of
photons in the halo are quickly approaching to a locally thermalized state
around the resonant frequency, even when the mean intensity of the radiation is
highly time-dependent. Since initial conditions are forgotten during the
thermalization, some features of the flux, such as the two peak structure of
its profile, actually are independent of the intrinsic width and time behavior
of the central source, if the emergent photons are mainly from photons in the
thermalized state. In this case, the difference $|\nu_{\pm}-\nu_0|$, where
$\nu_{\pm}$ are the frequencies of the two peaks of the flux, cannot be less
than $2$ times of Doppler broadening. We then study the radiative transfer in
the case where the light emitted from the central source is a flash. We
calculate the light curves of the flux from the halo. It shows that the flux is
still a flash. The time duration of the flash for the flux, however, is
independent of the original time duration of the light source but depends on
the optical depth of the halo. Therefore, the spatial transfer of resonant
photons is a diffusion process, even though it is not a purely Brownian
diffusion. This property enables an optically thick halo to trap and store
thermalized photons around $\nu_0$ for a long time after the cease of the
central source emission. The photons trapped in the halo can yield delayed
emission, of which the profile also shows typical two peak structure as that
from locally thermalized photons. Possible applications of these results are
addressed.
