The GROND gamma-ray burst sample

II. Fireball parameters for four gamma-ray burst afterglows^⋆

¹ Max-Planck Institut für extraterrestrische Physik, Giessenbachstr. 1, 85748 Garching, Germany
² Department of Physics, University of Bath, Building 3 West, Bath BA2 7AY, UK

^⋆⋆⋆ Corresponding authors: jcg@mpe.mpg.de, ps2018@bath.ac.uk, hjve20@bath.ac.uk

Received: 23 March 2025
Accepted: 7 July 2025

Abstract

Afterglows of gamma-ray bursts (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 seven-channel simultaneous optical and near-infrared imager at the 2.2 m telescope of the Max-Planck Society at ESO/La Silla, a dedicated GRB afterglow observing program was conducted between 2007 and 2016. Here, we combine GROND observations of four particularly well-sampled GRBs with public Swift/XRT data as well as sub-millimetre and radio data from both, our own and other groups’ programmes, 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 we find evidence for its origin through shock amplification of the magnetic field of the circumburst medium.