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 opticalflare emission detected by the 25 cm TAROT robotic telescope during the prompthigh-energy emission and the early afterglow. We combine our data withsimultaneous observations performed in X-rays and we analyze the broad-bandspectrum. These observations lead us to emphasize the similarity of GRB 050904with GRB 990123, a remarkable gamma-ray burst whose optical emission reached9th magnitude. While GRB 990123 was, until now, considered as a unique event,this observation suggests the existence of a population of GRBs which have verylarge isotropic equivalent energies and extremely bright optical counterparts.The luminosity of these GRBs is such that they are easily detectable throughthe entire universe. Since we can detect them to very high redshift even withsmall aperture telescopes like TAROT, they will constitute powerful tools forthe exploration of the high-redshift Universe and might be used to probe thefirst generation of stars.Comment: 9 pages, 3 figures. Accepted in ApJ