  GRB 080913, with a spectroscopically determined redshift of z=6.7, was the
record holder of the remotest 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 physics
of GRBs during the cosmic re-ionization epoch. We here present a detailed
numerical fit to the multi-wavelength data of the optical afterglow of GRB
080913 and then constrain its circum-burst 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.
