Aims. We present the time-resolved optical emission of GRB 060111B during its prompt phase, measured with the TAROT robotic observatory. This is the first time that the optical emission from a gamma-ray burst has been continuously monitored with a temporal resolution of a few seconds during the prompt gamma-ray phase.
Methods. The temporal evolution of the prompt optical emission at the level of several seconds is used to provide a clue to the origin of this emission.
Results. The optical emission was found to decay steadily from our first measure, 28 s after the trigger, in contrast to the gamma-ray emission, which exhibits strong variability at the same time. This behaviour strongly suggests that the optical emission is due to the reverse shock.