X‐Ray Lines from Gamma‐Ray Bursts

X-ray lines have been recently detected in the afterglows of a few gamma-raybursts. We derive constraints on the physical conditions in the line-emittinggas, using as an example the multiple K$_\alpha$ lines detected by Reeves etal. (2002) in GRB 011211. We argue that models previously discussed in theliterature require either a very extreme geometry or too much mass in theline-emitting region. We propose a new model in which gamma-rays and radiationfrom the early x-ray afterglow are back-scattered by an electron-positron pairscreen at a distance of about $10^{14}-10^{15}$ cm from the source andirradiate the expanding outer layers of the supernova ejecta, thereby producingx-ray lines. The model suffers from fewer problems compared to previous models.It also has the advantage of requiring only a single explosion to produce boththe GRB and the supernova ejecta, in contrast to most other models for thelines which require the supernova to go off days or weeks prior to the GRB. Themodel, however, has difficulty explaining the $>10^{48}$ ergs of energy emittedin the x-ray lines, which requires somewhat extreme choices of modelparameters. The difficulties associated with the various models are notparticular to GRB 011211. They are likely to pose a problem for any GRB withx-ray lines.Comment: 19 pages, 1 figure, submitted to Ap.