The early afterglow and magnetized ejecta present in GRB 110731A

TL;DR

This paper models the early afterglow of GRB 110731A using a leptonic wind afterglow framework, explaining multiwavelength observations and indicating magnetized ejecta with a high Lorentz factor.

Contribution

It introduces a leptonic wind afterglow model for GRB 110731A that accounts for multiwavelength data and suggests magnetized ejecta with a Lorentz factor of about 520.

Findings

Spectral energy distribution fits wind afterglow model

LAT emission explained by synchrotron and SSC from shocks

Ejecta inferred to be magnetized with high Lorentz factor

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.