A STRONG OPTICAL FLARE BEFORE THE RISING AFTERGLOW OF GRB 080129

We report on GROND observations of a 40 sec duration (rest-frame) optical flare from GRB 080129 at redshift 4.349. The rise- and decay time follow a power law with indices +12 and-8, respectively, inconsistent with a reverse shock and a factor 10 5 faster than variability caused by ISM 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 from the optical to the near-infrared once every minute. The delay of the flare relative to the GRB, its spectral energy distribution as well as the ratio of pulse widths suggest it to arise from residual collisions in GRB outflows (Li & Waxman 2008). If this interpretation is correct and can be supported by more detailed modelling or observation in further GRBs, the delay measurement provides an independent, determination of the Lorentz factor Γ of the outflow. Subject headings: gamma-rays: bursts – radiation mechanisms: non-thermal

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