SIMBAD references

Iron spectral features are thought to be the best tracers of progenitors of gamma-ray bursts (GRBs). The detections of spectral features such as an iron line and/or the radiative recombination edge and continuum (RRC) were reported in four X-ray afterglows of GRBs. However, burst for burst, their properties were different from each other. For example, the Chandra observation of GRB 991216 detected a strong H-like iron line together with the RRC. With ASCA, on the other hand, Yoshida and coworkers detected only the strong RRC in GRB 970828. Since it is difficult to produce the strong RRC, we have to consider a special condition for the line- and/or RRC-forming regions. In this Letter, we point out the possibility of a ``nonequilibrium ionization state'' for the line- and RRC-forming regions.