On the formation of Lyman $\alpha$ emission from resonantly scattered
  continuum photons of GRB's afterglow
  We study the formation and evolution of the spectral imprint of resonantly
scattered Lyman $\alpha$ photons in the context of GRB's continuum optical
afterglow. Based on an analytic model and a complete treatment of all the
scatterings using Monte Carlo simulations, we found that the flux of the
Ly$\alpha$ emission is mainly contributed by photons which are scattered only
once. The flux is of order $10^{-4}$ to $10^{-9}$ relative to the undecayed
maximum flux of the transmitted continuum, making the feature negligible but
potentially observable. If not obscured by host galaxy's DLA or intergalactic
neutral hydrogen, the feature may appear sometime from one hour to several
years when the directly transmitted light has faded away.This scattered
emission feature can be distinguished from Ly$\alpha$ photons of other origins
by its luminosity evolution, and by its gradual narrowing of profile with time.
The typical time scale for spectral variance is that of the light crossing time
of a hydrogen clump close to the GRB.
