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 Gamma of the outflow.