The GROND GRB sample: II. Fireball parameters for four GRB afterglows

TL;DR

This study analyzes four well-sampled GRB afterglows using multi-wavelength data to determine fireball model parameters, revealing wind environments and magnetic field evolution consistent with shock amplification.

Contribution

It provides detailed fireball parameters for four GRBs using combined optical, X-ray, sub-mm, and radio data, a comprehensive approach not previously applied to this sample.

Findings

All four GRBs exploded into wind environments.

Magnetic field strength evolution inferred from data.

Evidence for shock amplification of magnetic fields.

Abstract

Afterglows of GRBs are, in general, well described by the fireball model. Yet, deducing the full set of model parameters from observations without prior assumptions has been possible for only a handful of GRBs. With GROND, a 7-channel simultaneous optical and near-infrared imager at the 2.2m telescope of the Max-Planck Society at ESO/La Silla, a dedicated gamma-ray burst (GRB) afterglow observing program was performed between 2007 and 2016. Here, we combine GROND observations of four particularly well-sampled GRBs with public Swift/XRT data and partly own sub-mm and radio data to determine the basic fireball afterglow parameters. We find that all four bursts exploded into a wind environment. We are able to infer the evolution of the magnetic field strength from our data, and find evidence for its origin through shock amplification of the magnetic field of the circumburst medium.