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Recent detections of GeV photons in a few GRBs by Fermi-LAT have led tostrong constraints on the bulk Lorentz factor in GRB outflows. To avoid a largegamma gamma optical depth, minimum values of the Lorentz factor are estimatedto be as high as 800-1200 in some bursts. Here we present a detailedcalculation of the gamma gamma optical depth taking into account both thegeometry and the dynamics of the jet. In the framework of the internal shockmodel, we compute lightcurves in different energy bands and the correspondingspectrum and we show how the limits on the Lorentz factor can be significantlylowered compared to previous estimates.

Do Fermi-LAT observations really imply very large Lorentz factors in GRB outflows ?