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

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

Dec 2011

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2011sf2a.conf..593h&link_type=abstract

SF2A-2011: Proceedings of the Annual meeting of the French Society of Astronomy and Astrophysics Eds.: G. Alecian, K. Belkacem,

Astronomy and Astrophysics

Astrophysics

Gamma-Ray Bursts, Grb 080916C, Radiative Transfer, Non Thermal Radiation Mechanisms

Scientific paper

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 have been 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 light curves 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. Our detailed model of the propagation of high energy photons in GRB outflows is also appropriate to study many other consequences of γγ annihilation in GRBs: (i) the γγ cutoff transition in a time-integrated spectrum is expected to be closer to a power-law steepening of the spectrum than to a sharp exponential decay; (ii) the temporal evolution of the γγ opacity during a burst favors a delay between the MeV and GeV light curves; (iii) for complex GRBs, the γγ opacity suppress the shortest time-scale features in high energy light curves (above 100 MeV). Finally we also consider GRB scenarii where MeV and GeV photons are not produced at the same location, showing that the γγ opacity could be further lowered, reducing even more the constraint on the minimum Lorentz factor.

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