A strong optical flare before the rising afterglow of GRB 080129

Understand

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 sub-components, but also provide a spectral energy distribution from the optical to the near-infrared once every minute.