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2 Model assumptions

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, $E_0 = 10^{54} \, E_{54}$ erg, the initial bulk Lorentz factor $\Gamma_0$, 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 $n_{\rm ext} (r) = n_0 \, (r / r_{\rm d})^{-\eta}$ is assumed to have a power-law profile in distance from the burst source determined by the density n0 at the deceleration radius $r_{\rm d}$ and the power-law index $\eta$.

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 $n_0 \mathrel{\mathchoice {\vcenter{\offinterlineskip\halign{\hfil
$\displaystyl...
 ...offinterlineskip\halign{\hfil$\scriptscriptstyle ...  cm-3, recombination is negligible.


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