Iron K alpha emission from X-ray reflection: Predictions for gamma-ray burst models

Recent observations of several gamma -ray burst (GRB) afterglows have shown evidence for a large amount of X-ray line-emitting material, possibly aris ing from ionized iron. A significant detection of an X-ray spectral feature , such as that found in the Chandra observation of GRB 991216, may provide important constraints on the immediate environment of the burst and hence o n progenitor models. The large Fe K alpha equivalent widths inferred from t he X-ray observations favor models in which the line is produced when the p rimary X-ray emission from the source strikes Thomson-thick material and Co mpton scatters into our line of sight. We present such reflection spectra h ere, computed in a fully self-consistent manner, and discuss the range of i onization parameters that may be relevant to different models of GRBs. We a rgue that the presence of a strong hydrogen-like K alpha line is unlikely, because Fe XXVI photons would be trapped resonantly and removed from the li ne core by Compton scattering. In contrast, a strong narrow emission line f rom He-like Fe XXV is prominent in the model spectra. We briefly discuss ho w these constraints may affect the line energy determination in GRB 991216.