A strong optical flare before the rising afterglow of GRB 080129

We report on GROND observations of a 40 s duration (rest-frame) optical flare from GRB 080129 at redshift 4.349. The rise and decay times follow a power law with indices +12 and –8, respectively, inconsistent with a reverse shock and a factor 105 faster than variability caused by interstellar material interaction. While optical flares have been seen in the past (e.g., GRB 990123, 041219B, 060111B, and 080319B), for the first time, our observations not only resolve the optical flare into subcomponents, but also provide a spectral energy distribution (SED) from the optical to the near-infrared once every minute. The delay of the flare relative to the gamma-ray burst (GRB), its SED as well as the ratio of pulse widths suggest it to arise from residual collisions in GRB outflows. If this interpretation is correct and can be supported by a more detailed modeling or observation in further GRBs, the delay measurement provides an independent determination of the Lorentz factor Γ of the outflow.