Nonequilibrium ionization states of gamma-ray burst environments

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 ir on line and/or the radiative recombination edge and continuum (RRC) were re ported in four X-ray afterglows of GRBs. However, burst for burst, their pr operties were different from each other. For example, the Chandra observati on 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 stron g RRC in GRB 970828. Since it is difficult to produce the strong RRC, we ha ve to consider a special condition for the line- and/or RRC-forming regions . In this Letter, we point out the possibility of a "nonequilibrium ionizat ion state" for the line- and RRC-forming regions.