Letter to the Editor

Continuous optical monitoring during the prompt emission of GRB 060111B

¹ CESR, Observatoire Midi-Pyrénées (CNRS - UPS), BP 4346, 31028 Toulouse Cedex 04, France e-mail: klotz@cesr.fr
² Observatoire de Haute-Provence, 04870 Saint Michel l'Observatoire, France
³ IASF - Roma/INAF, via fosso del cavaliere 100, 00133 Roma, Italy
⁴ LATT, Observatoire Midi-Pyrénées (CNRS - UPS), 14 avenue E. Belin, 31400 Toulouse, France
⁵ Observatoire de Genève, 1290 Sauverny, Switzerland

Received: 8 March 2006
Accepted: 28 March 2006

Abstract

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.