Multiwavelength observations of GRB 110731A: GeV emission from onset to afterglow

TL;DR

This paper presents multiwavelength observations of GRB 110731A, revealing detailed temporal and spectral features from prompt emission to afterglow, including GeV emission consistent with forward shock models and a high Lorentz factor estimate.

Contribution

First simultaneous multi-epoch GeV, X-ray, and optical data for a GRB, enabling comprehensive analysis of prompt and afterglow emission mechanisms.

Findings

Extended GeV emission linked to afterglow from forward shock

Bulk Lorentz factor estimated at 500-550

Emission features similar to other bright LAT GRBs

Abstract

We report on the multiwavelength observations of the bright, long gamma-ray burst \GRB, by the \Fermi and \Swift observatories, and by the MOA and GROND optical telescopes. The analysis of the prompt phase reveals that \GRB shares many features with bright Large Area Telescope bursts observed by \Fermi during the first 3 years on-orbit: a light curve with short time variability across the whole energy range during the prompt phase, delayed onset of the emission above 100 MeV, extra power law component and temporally extended high-energy emission. In addition, this the first GRB for which simultaneous GeV, X-ray, and optical data are available over multiple epochs beginning just after the trigger time and extending for more than 800 s, allowing temporal and spectral analysis in different epochs that favor emission from the forward shock in a wind-type medium. The observed temporally extended GeV emission is most likely part of the high-energy end of the afterglow emission. Both the single-zone pair transparency constraint for the prompt signal, and the spectral and temporal analysis of the forward shock afterglow emission, independently lead to an estimate of the bulk Lorentz factor of the jet $\Gamma\sim$ 500 - 550.