One of the most energetic gamma-ray burst GRB 110731A was observed from optical to GeV energy range .
Previous analysis on the prompt phase revealed similarities with the Large Area Telescope ( LAT ) bursts observed by Fermi : i ) a delayed onset of the high-energy emission ( > 100 MeV ) , ii ) a short-lasting bright peak at later times and iii ) a temporally extended component from this phase and lasting hundreds of seconds .
Additionally to the prompt phase , multiwavelength observations over different epochs 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 of its progenitor .
We apply this model to interpret the temporally extended LAT emission and the brightest LAT peak exhibited by the prompt phase of GRB 110731A .
Additionally , using the same set of parameters , we describe the multiwavelength afterglow observations .
The origin of the temporally extended LAT , X-ray and optical flux is explained through synchrotron radiation from the forward shock and the brightest LAT peak is described evoking the synchrotron self-Compton emission from the reverse shock .
The bulk Lorentz factor required in this model ( \Gamma \simeq 520 ) lies in the range of values demanded for most LAT-detected gamma-ray bursts .
We show that the strength of the magnetic field in the reverse-shock region is \sim 50 times stronger than in the forward-shock region .
This result suggests that for GRB 110731A , the central engine is likely entrained with strong magnetic fields .