Lorentz factor constraint from the very early external shock of the gamma-ray burst ejecta

While it is generally agreed that the emitting regions in gamma-ray bursts move ultra-relativistically towards the observer, different estimates of the initial Lorentz factors, Gamma(0), lead to different at times conflicting estimates. We show here that the quiet periods in which the signals goes down below the instrumental thresholds put strong upper limits on the values of Gamma(0). According to the standard internal-external shock model, an external shock should develop during the prompt stage. This external shock radiates in the hard X-rays to soft gamma-rays bands and this emission should be seen as a smooth background signal. The observed deep minima indicate that this contribution is negligible. This limits, in turn, Gamma(0). We obtain upper limits on Gamma(0) for several bursts with typical values around hundreds. We compare these values with those obtained by the other methods, which typically yield lower limits. The results are marginally consistent leaving only a narrow range of allowed values for Gamma(0).