Fig. 4

High-energy spectral extrapolation of the GRB 080721. The blue horizontal line shows the derived expectation for the GRB flux in the band 30 MeV−3 GeV, adopting a spectral index β = 2.43, while the red line is the UL flux estimated with the Helene method. Since the UL is slightly below the expectations a steeper spectral index is expected.
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