Li 2008 GeV emission in Residual internal shocks

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arXiv:0810.2932 [ps, pdf, other]

Title: Prompt GeV Emission from Residual Collisions in GRB Outflows

Authors: Zhuo Li (PKU)

Comments: 4 pages, 1 figure

Subjects: Astrophysics (astro-ph)

The gamma-rays from gamma-ray bursts (GRBs) are believed to be produced by internal shocks driven by small timescale, ~1 ms, variation in the GRB outflows, and a pair-production spectral cutoff, ~1(\Gamma/300)^6 GeV (\Gamma is the bulk Lorentz factor of the outflow), is generally expected in the GRB spectrum. However, the observed optical flashes accompanying GRBs suggest that the delayed residual collisions at large radii continue to accelerate electrons. We show here that the inverse-Compton scattering of the prompt gamma-rays by these residual internal shock electrons leads to a high energy emission beyond the previously thought spectral cutoff, in agreement with the previous detections of GeV photons by EGRET in several GRBs in conjunction with MeV emission. We expect a spectral steepening in the connection between the primary and residual internal shock emission at the previously thought spectral cutoff, and expect systematic time delays of high energy photons relative to MeV emission, the discovery of which would provide stringent constraint on the outflow properties, but requires large enough collection of high energy photons by, e.g., GLAST and AGILE.