We simulate a relativistic blast wave expanding into and
photoionizing a stationary external medium. The evolution
of the blast wave and of the GRB spectrum is represented by
the analytic parametrization of
Dermer et al. (1999).
The model is mainly determined by the total explosion energy,
erg, the initial bulk Lorentz
factor
, and the index g characterizing the radiative
regime of the blast wave evolution, with g = 3/2 corresponding
to a non-radiative (adiabatic) blast wave, while g = 3 describes
a fully radiative blast wave. The CBM density distribution
is assumed to
have a power-law profile in distance from the burst source determined
by the density n0 at the deceleration radius
and the power-law
index
.
We solve the time-dependent radiation transfer and photoionization
problem for H, He, C, N, O, Ne, Mg, Si, S, Ar, Ca, Fe, and Ni. For
the results presented in this paper we assume standard solar-system
element abundances. We use the photoionization cross sections for
all subshells of all elements using the relevant subroutines of
the XSTAR code
(Kallman & Krolik 1998).
Auger and radiative
transitions following inner-shell photoionization events are
calculated using the tables of
Kaastra & Mewe (1993).
We account for the light-travel time delay of
fluorescence line emission. Since we assume densities of
cm-3, recombination is negligible.
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