M. Böttcher, and C. L. Fryer,
Astrophysical Journal, 547, 338 (2001)
We investigate the potentially observable prompt or delayed
X-ray spectral features from the currently popular gamma-ray
burst (GRB) models. During the evolution of many GRB progenitors,
a disk around the central GRB source is produced. Shock heating
as the GRB ejecta collide with the disk may produce observable
X-ray features. We first summarize predictions deduced from
previous calculations which invoke photoionization and relativistic
blast waves. We then calculate the quasi-thermal X-ray line features
produced assuming the ejecta are nonrelativistic (which is
more likely for the disk interactions of many GRB models).
In the framework of the Hypernova/Collapsar model, delayed
(a few days - several months after the GRB) bursts of
line-dominated, thermal X-ray emission may be expected.
The He-merger scenario predicts similar X-ray emission
line bursts 
a few
days after the GRB. These X-ray signatures should be
observable with Chandra and XMM-Newton out
to at least z ~ 1. Weak emission line features a few days after the GRB may also
result from the supranova GRB scenario. In all three cases,
significant X-ray absorption features, in particular during
the prompt GRB phase, are expected. No significant X-ray
spectral features might result from compact-object binary
mergers.