On the X-Ray Lines in the Afterglows of Gamma-Ray Bursts

The observation of X-ray lines in the afterglow of GRB 011211 has been reported, and challenged. The lines were interpreted as blueshifted X-rays characteristic of a set of photoionized "metals," located in a section of a supernova shell illuminated by a gamma-ray burst (GRB) emitted a couple of days after the supernova explosion. We show that the most prominent reported lines coincide with the ones predicted in the "cannonball" model of GRBs. In this model, the putative signatures are hydrogen lines, boosted by the (highly relativistic) motion of the cannonballs (CBs). The corresponding Doppler boost can be extracted from the fit to the observed I-, R-, and V-band light curves of the optical afterglow of GRB 011211, so that, since the redshift is also known, the line energies are (in the CB model) predicted. We also discuss other GRBs of known redshift which show spectral features generally interpreted as Fe lines, or Fe recombination edges. The ensemble of results is very encouraging from the CB modelʹs point of view, but the data on each individual GRB are not good enough to draw (any) objectively decisive conclusions. We outline a strategy for X-ray observers to search for lines which, in the CB model, move predictably from higher to lower energies.