Iron Kα Emission from X-Ray Reflection: Predictions for Gamma-Ray Burst Models

Recent observations of several gamma-ray burst (GRB) afterglows have shownevidence for a large amount of X-ray line emitting material, possibly arisingfrom ionized iron. A significant detection of an X-ray spectral feature, suchas that found in the Chandra observation of GRB 991216, may provide importantconstraints on the immediate environment of the burst and hence on progenitormodels. The large Fe K-alpha equivalent widths inferred from the X-rayobservations favor models in which the line is produced when the primary X-rayemission from the source strikes Thomson-thick material and Compton scattersinto our line of sight. We present such reflection spectra here, computed in afully self-consistent manner, and discuss the range of ionization parametersthat may be relevant to different models of GRBs. We argue that the presence ofa strong hydrogen-like K-alpha line is unlikely, because Fe-XXVI photons wouldbe trapped resonantly and removed from the line core by Compton scattering. Incontrast, a strong narrow emission line from He-like Fe-XXV is prominent in themodel spectra. We briefly discuss how these constraints may affect the lineenergy determination in GRB 991216.Comment: 8 pages, 3 figures, Ap.J. Letters accepte