Testing GRB models with the strange afterglow of GRB 090102

We present the observations of the afterglow of gamma-ray burst GRB 090102. We use optical data taken by the TAROT, REM, GROND, Palomar and NOT telescopes, and X-ray data taken by the XRT instrument on board the Swift spacecraft. This event features an unusual light curve. In X-rays, it presents a very monotonic decrease with no hint of temporal break from 0.005 to 6 days after the burst. In optical, the light curve presents a flattening after 1 ks. Before this break, the optical light curve is steeper than the X-ray one. In optical, no further break is observed up to 10 days after the burst. We tried to explain these observations in light of the standard fireball model, but we failed to do so. We then investigated several other models, like the cannonball model. We find that the explanation of the broad band data by any model requires a strong fine tuning when taking into account both optical and X-ray bands.