Abstract
We analyze the >100-MeV data for three gamma-ray bursts (GRBs) detected by the Fermi satellite (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 behaviour of the >100 MeV data, and late-time X-ray and optical data, and find them consistent with the so-called closure relations for the external forward shock radiation. (2) We calculate the expected external forward shock synchrotron flux at 100 MeV, which is essentially a function of the total energy in the burst alone, and it matches the observed flux value. (3) We determine the external forward shock model parameters using the >100 MeV data (a very large phase space of parameters is allowed by the high-energy data alone), and for each point in the allowed parameter space we calculate the expected X-ray and optical fluxes at late times (hours to days after the burst) and find these to be in good agreement with the observed data for the entire parameter space allowed by the >100 MeV data. (4) We calculate the external forward shock model parameters using only the late-time X-ray, optical and radio data and from these estimate the expected flux at >100 MeV at the end of the sub-MeV burst (and at subsequent times) and find that to be entirely consistent with the high-energy data obtained by Fermi/LAT. The ability of a simple external forward shock, with two empirical parameters (total burst energy and energy in electrons) and two free parameters (circumstellar density and energy in magnetic fields), to fit the entire data from the end of the burst (1–50 s) to about a week, covering more than eight decades in photon frequency –>102 MeV, X-ray, optical and radio – provides compelling confirmation of the external forward shock synchrotron origin of the >100 MeV radiation from these Fermi GRBs. Moreover, the parameters determined in points (3) and (4) show that the magnetic field required in these GRBs is consistent with shock-compressed magnetic field in the circumstellar medium with pre-shocked values of a few tens of μG.