GRB 080913,with a spectroscopically determined redshift of z=6.7,was the record holder for being the most remote stellar object before the discovery of the recent gamma-ray burst GRB 090423,whose redshift is about 8.2.The gradually accumulated high redshift GRB sample has shed light on the origin and physical properties of GRBs during the cosmic re-ionization epoch.Here,we present a detailed numerical fit to the multi-wavelength data of the optical afterglow of GRB 080913 and then constrain its circumburst environment and the other model parameters.We conclude that the late optical/X-ray plateau at about one day since the burst is due to the Poynting-flux dominated injection from the central engine which is very likely a massive spinning black hole with super strong magnetic fields.
LIU XueWen,WU XueFeng & LU Tan Purple Mountain Observatory,Chinese Academy of Sciences,Nanjing 210008,China
In the fireball model, it is more physically realistic that ganuna-ray burst （GRB） ejecta have a range of bulk Lorentz factors （assuming M ∝ Г^-8）. The low Lorentz factor part of the ejecta will catch up with the high Lorentz factor part when the latter is decelerated by the surrounding medium to a comparable Lorentz factor. Such a process will develop a long-lasting weak reverse shock until the whole ejecta are shocked. Meanwhile, the forward shocked materials are gradually supplied with energy from the ejecta that are catching-up, and thus the temporal decay of the forward shock emission will be slower than that without an energy supply. However, the reverse shock may be strong. Here, we extend the standard reverse-forward shock model to the case of radially nonuniform ejecta. We show that this process can be classified into two cases： the thick shell case and the thin shell case. In the thin shell case, the reverse shock is weak and the temporal scaling law of the afterglow is the same as that in Sad ＆ Meszaros （2000）. However, in the thick shell case, the reverse shock is strong and thus its emission dominates the afterglow in the high energy band. Our results also show slower decaying behavior of the afterglow due to the energy supply by low Lorentz factor materials, which may help the understanding of the plateau observed in the early optical and X-ray afterglows.
