@inproceedings{f67e6b62c3884820b1885c1c8177f913,
title = "Do fermi-LAT observations really imply very large Lorentz factors in GRB outflows?",
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 γγ 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 γγ 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.",
keywords = "γ-ray bursts, γ-ray sources, γγ annihilation",
author = "R. Hasco{\"e}t and V. Vennin and F. Daigne and R. Mochkovitch",
year = "2011",
month = sep,
day = "12",
doi = "10.1063/1.3621739",
language = "English",
isbn = "9780735409163",
series = "AIP Conference Proceedings",
pages = "67--70",
booktitle = "Gamma Ray Bursts 2010, GRB 2010",
note = "Gamma Ray Bursts 2010, GRB 2010 ; Conference date: 01-11-2010 Through 04-11-2010",
}