Detection of a very bright optical flare from the gamma-ray burst GRB 050904 at redshift 6.29

In this Letter, we discuss the flux and the behavior of the bright optical flare emission detected by the 25 cm TAROT robotic telescope during the prompt high-energy emission and the early afterglow. We combine our data with simultaneous observations performed in X-rays, and we analyze the broadband spectrum. These observations lead us to emphasize the similarity of GRB 050904 with GRB 990123, a remarkable gamma-ray burst whose optical emission reached 9th magnitude. While GRB 990123 was, until now, considered as a unique event, this observation suggests the existence of a population of GRBs that have very large isotropic equivalent energies and extremely bright optical counterparts. The luminosity of these GRBs is such that they are easily detectable through the entire universe. Since we can detect them to very high redshift even with small-aperture telescopes like TAROT, they will constitute powerful tools for the exploration of the high-redshift universe and might be used to probe the first generation of stars.