Abstract
Gamma-ray bursts (GRBs) are known to have the most relativistic jets, with initial Lorentz factors in the order of a few hundreds. Many GRBs display an early X-ray light-curve plateau, which was not theoretically expected and therefore puzzled the community for many years. Here, we show that this observed signal is naturally obtained within the classical GRB fireball model, provided that the initial Lorentz factor is rather a few tens, and the expansion occurs into a medium-low density wind. The range of Lorentz factors in GRB jets is thus much wider than previously thought and bridges an observational gap between mildly relativistic jets inferred in active galactic nuclei, to highly relativistic jets deduced in few extreme GRBs. Furthermore, long GRB progenitors are either not Wolf-Rayet stars, or the wind properties during the final stellar evolution phase are different than at earlier times. Our model has predictions that can be tested to verify or reject it in the future, such as lack of GeV emission, lack of strong thermal component and long (few seconds) variability during the prompt phase characterizing plateau bursts.