Spectral features, in most cases time-variable, have been reported to be detected in the 0.5-10 keV X-ray spectra for several GRBs (e.g.,  for a critical review). While in most cases only one single line has been seen, which usually has been associated to iron, there are few cases (GRB 011211, 030227) for which several emission lines were seen. There are, still, no two GRBs for which the same line(s) have been detected. In all cases, the significance of these lines is marginal, despite the improved sensitivity and reduced delay in observing them with, e.g., XMM- Newton as compared to ASCA and BeppoSAX, and therefore the issue is controversial. One way to explain the widely different lines is to assume that 56Ni is mainly synthesized in the jet due to neutrino heating . In this case, the 56Ni production is very sensitive to the energy deposition rate, i.e., a long energy deposition rate would synthesize little 56Ni, but Si and S instead. If this is true, there should be a correlation between the line features in the X-ray afterglows and the duration of the GRB.
In more general terms, the detection of X-ray lines offers a direct probe of the hot and potentially dense environment immediately surrounding the GRB. However, the interpretation of these lines or line complexes requires extreme conditions on either the parameters of the surrounding medium or its geometry. This and the diversity of emission lines are the reason why until now there is no adequate model to explain all observed features.
Beyond studying the GRB environment, X-ray lines are also of prime importance in determining the nature of their progenitors as well as remnants.
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