On the Transient Fe K-Edge in the Prompt Emission of GRB 990705

Abstract.

A transient Fe absorption edge during the prompt phase of a GRB has recently been observed for the first time. This is evidence for photoionization of the GRB environment by the burst radiation and allows diagnostics of the density structure and element abundances in the vicinity of the burst. We model the time-dependent photoionization and X-ray radiation transport, and deduce constraints on the size and composition of the photoionized region responsible for the absorption feature. We find that the intervening materialm ust contain $\sim 44\Omega M_{\bigodot}$ of iron within $\sim 1.3$ pc of the burst source, assuming the measured best-fit $\sim 75$-fold overabundance of iron. Possible solutions to reduce the required amount of iron are briefly discussed.