The early afterglow and magnetized ejecta present in GRB 110731A

Abstract

One of the most energetic gamma-ray bursts GRB 110731A, was observed from optical to GeV energy range by Fermi and Swift Observatories, and by the MOA and GROND optical telescopes. The multiwavelength observations over different epochs (from trigger time to more than 800 s) showed that the spectral energy distribution was better fitted by a wind afterglow model. We present a leptonic model based on an early afterglow that evolves in a stellar wind to describe the multiwavelength light curves observations. In particular, the origin of the LAT emission is explained through the superposition of synchrotron radiation from the forward shock and synchrotron self-Compton emission from the reverse shock. The bulk Lorentz factor required in this model is $\Gamma\simeq520$ and the result suggests that the ejecta must be magnetized.