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The 2009 Fermi Symposium

High Energy Emission from Short GRBs

Alessandra Corsi, IASF/INAF-Roma


Recent detection by the Agile and Fermi satellites of substantial high energy emission from short Gamma-Ray Bursts (GRBs), calls for a reconsideration of these bursts as high energy sources. Such results are in fact surprising in the light of the expectations from before the launch of Agile/Fermi, when high energy emission was more likely expected in coincidence with long GRBs. Motivated by these recent results, we analyze GRB 081024B and GRB 090510 observations in the context of the most popular theoretical models, as synchrotron and synchrotron self-Compton (SSC) emission from prompt/delayed internal shocks (IS), or from the external shock (ES). In the case of GRB 081024B, we show that the long-lasting high energy tail observed by the Fermi/LAT cannot be explained as synchrotron or SSC emission from the IS generating the prompt emission. On the other hand, SSC emission from late IS or from the ES generating the afterglow are both able to accommodate the observations for this burst. In the case of GRB 090510, we demonstrate that the spectral evolution observed by AGILE and Fermi during the first second of the burst, cannot be explained by a single process, such as the IS model. Two independent components are required. We consider the possibility of linking the observed high energy tail to emission from the ES. We present the fireball model prediction and compare it to broad-band data, from optical to GeV range.