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

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arxiv 1101.3889 v2 pith:XQOD5JGB submitted 2011-01-20 astro-ph.HE

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

classification astro-ph.HE
keywords gammalorentzfactordepthfermi-latlargeopticaloutflows
verification ladder T0 review T1 audit T2 compute T3 formal T4 reserved
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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.

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