Fe K$\alpha$ Emission from X-ray Reflection: Predictions for Gamma-Ray Burst Models

D.R. Ballantyne & E. Ramirez-Ruiz (Institute of Astronomy, University of Cambridge, U.K.),

Abstract

The recent discovery of Fe K$\alpha$ line features in the X-ray afterglows of several $\gamma$-ray bursts (GRBs), particularly the Chandra observation of GRB 991216, may provide important constraints on the immediate environment of the burst and hence on progenitor models. The large Fe K$\alpha$ equivalent widths inferred from the X-ray observations favor models in which the line is produced when the primary X-ray emission from the source strikes Thomson-thick material and Compton scatters into our line of sight. We present such reflection spectra here, computed in a fully self-consistent manner, and discuss the range of ionization parameters that may be relevant to different models of GRBs. We argue that the presence of a strong hydrogen-like K$\alpha$ line is unlikely because Fe  XXVI photons are likely to be resonantly trapped and removed from the line core by Compton scattering. In contrast, a strong narrow emission line from He-like Fe  XXV is prominent in the model spectra. We briefly discuss how these constraints may affect the line energy determination in GRB 991216.

CATEGORY: SOLAR SYSTEM AND MISC