Letter to the Editor

X-ray spectral diagnostics of the immediate environment of GRB 991216

¹ Institute of Astronomy, University of Cambridge, Madingley Road, CB3 0HA Cambridge, UK e-mail: drb,enrico,lazzati@ast.cam.ac.uk
² Istituto di Astrofisica Spaziale, CNR, Via del Fosso del Cavaliere 100, 00133 Roma, Italy e-mail: piro@ias.rm.cnr.it

Corresponding author: D. Ballantyne, drb@ast.cam.ac.uk

Received: 15 April 2002
Accepted: 6 June 2002

Abstract

The recent report of iron line features in the afterglow of the gamma-ray burst (GRB) 991216 has important implications for the properties of the radiating material and hence the nature of the immediate burst environment. We argue that the putative strong Fe emission line can be attributed to the reflected emission from Thomson-thick matter which is illuminated by a power-law continuum. The ionization parameter of the material (i.e., the flux to density ratio) is around 10³, resulting in a Fe Kα line from He-like iron. A supersolar abundance of iron is not required by the data. Interestingly, the ionizing continuum must be harder than the observed one. We interpret this as due to the fact that the observed continuum is dominated by the standard blast wave emission, while the line is produced by reprocessing material located at much smaller radii.