  The recent detection of a transient absorption feature in the X-ray prompt
emission of GRB 990705 showed the importance of such observations in the
understanding of gamma-ray bursts and their progenitors. We investigate the
time dependence of photoionization edges during the prompt emission of bursts
in different environments. We show that their variability can be used to infer
the density and geometry of the surrounding medium, giving important clues to
unveil the nature of the burst progenitor.
