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

TL;DR

This paper reevaluates the constraints on the Lorentz factor in GRB outflows from Fermi-LAT observations, showing that more realistic models can significantly lower previous estimates of these factors.

Contribution

It provides a detailed calculation of gamma gamma optical depth considering jet geometry and dynamics, challenging previous high Lorentz factor constraints.

Findings

Lowered Lorentz factor limits compared to previous estimates

Detailed modeling of jet geometry and dynamics effects

Revised interpretation of Fermi-LAT GRB observations

Abstract

Recent detections of GeV photons in a few GRBs by Fermi-LAT have led to strong constraints on the bulk Lorentz factor in GRB outflows. To avoid a large gamma gamma optical depth, minimum values of the Lorentz factor are estimated to be as high as 800-1200 in some bursts. Here we present a detailed calculation of the gamma gamma optical depth taking into account both the geometry and the dynamics of the jet. In the framework of the internal shock model, we compute lightcurves in different energy bands and the corresponding spectrum and we show how the limits on the Lorentz factor can be significantly lowered compared to previous estimates.