External forward shock origin of high energy emission for three GRBs detected by Fermi

Understand

We analyze the >100MeV data for 3 GRBs detected by Fermi (GRBs 080916C, 090510, 090902B) and find that these photons were generated via synchrotron emission in the external forward shock.

• We arrive at this conclusion by four different methods as follows.
• (1) We check the light curve and spectral behavior of the >100MeV data, and late time X-ray and optical data, and find them consistent with the closure relations for the external forward shock radiation.
• (2) We calculate the expected external forward shock synchrotron flux at 100MeV, and it matches the observed flux value.