M. Böttcher,
Astrophysical Journal, 539, 102 (2000)
The time and angle dependent line and continuum emission
from a dense torus around a cosmological gamma-ray burst
source is simulated, taking into account photoionization,
collisional ionization, recombination, and electron
heating and cooling due to various processes. The importance
of the hydrodynamical interaction between the torus and
the expanding blast wave is stressed. Due to the rapid
deceleration of the blast wave as it interacts with the dense
torus, the material in the torus will be illuminated by a
drastically different photon spectrum than observable through
a low-column-density line of sight, and will be heated by
the hydrodynamical interaction between the blast wave and
the torus. A model calculation to reproduce the Fe K
line emission observed in the X-ray afterglow of GRB 970508
is presented. The results indicate that ~ 10-4
M0 of iron must be concentrated in a region of
R < 10-3 pc. The illumination of the torus
material due to the hydrodynamic interaction of the blast
wave with the torus is the dominant heating and ionization
mechanism leading to the formation of the iron line. These
results suggest that misaligned GRBs may be detectable as
X-ray flashes with pronounced iron emission line features.