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

Understand

Recent detections of GeV photons in a few GRBs by Fermi-LAT imply huge bulk Lorentz factors to avoid a large gamma gamma optical depth at high energy.

• Estimates can be as high as Gamma ~ 1000 in the most extreme cases.
• This puts severe constraints on models of the central engine and the jet acceleration in GRBs.
• These estimates are however obtained from a simplified single zone model.