Gas opacity τ as a function of energy for the circumburst environment of a GRB for N_H = 10²³cm⁻². The green and red lines correspond to spatial resolution ,10²⁰cm⁻², respectively, and do not account for the gas absorption within each shell. The blue and purple lines (superimposed throughout most of the energy range) correspond to resolutions of ,10²¹ cm⁻² and account for absorption following Eq. 8.