  Absorption and reprocessing of Gamma-ray burst radiation in the environment
of cosmological GRBs can be used as a powerful probe of the elusive nature of
their progenitors. Although it is widely accepted that long-duration GRBs are
associated with the deaths of massive stars, at least two fundamentally
different scenarios concerning the final collapse are currently being
considered. Delayed reddened excesses in the optical afterglows of several GRBs
indicate that a supernova, possibly of Type Ic, takes place within a few days
of a GRB. This supports the collapsar model, where the core of a massive star
collapses promptly to a black hole. Variable X-ray features observed in the
prompt and afterglow spectra of several GRBs, suggest that a highly metal
enriched and dense shell of material surrounds the sources of GRBs. In some
cases, evidence for expansion of these shells with velocities of a substantial
fraction of the speed of light has been claimed. These observations have been
interpreted as support for the supranova model, where a massive star collapses
first to a supramassive neutron star, which later collapses to a black hole
following loss of rotational support. In this review paper, I will present a
brief overview of the current status of the observational evidence for X-ray
spectral features in GRBs, and discuss their implications for both the
collapsar and the supranova model.
