The GROND gamma-ray burst sample: II. Fireball parameters for four gamma-ray burst afterglows

K. Varela; J. Greiner; P. Schady; H. Van Eerten

doi:10.1051/0004-6361/202554710

The GROND gamma-ray burst sample: II. Fireball parameters for four gamma-ray burst afterglows

K. Varela, J. Greiner, P. Schady, H. Van Eerten

Max Planck Institut für Extraterrestrische Physik

Research output: Contribution to journal › Article › peer-review

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.

Original language	English
Article number	A11
Number of pages	27
Journal	Astronomy and Astrophysics
Volume	701
Early online date	2 Sept 2025
DOIs	https://doi.org/10.1051/0004-6361/202554710
Publication status	Published - 30 Sept 2025

Acknowledgements

We thank the referee for helpful comments. Part of the funding for GROND (both hardware as well as personnel) was generously granted from the Leibniz-Prize to Prof. G. Hasinger (DFG grant HA 1850/28-1). KV and JG are grateful for APEX support by K. Menten, A. Weiss, and F. Bertoldi. APEX is operated by the Max-Planck Institut für Radioastronomie, the European Southern Observatory, and the Onsala Space Observatory. This work made use of data supplied by the UK Swift Science Data Centre at the University of Leicester.

Funding

Open Access funding provided by Max Planck Society.

Keywords

Gamma-ray burst: general
Gamma-ray burst: individual: GRB 100418A
Gamma-ray burst: individual: GRB 110715A
Gamma-ray burst: individual: GRB 121024A
Gamma-ray burst: individual: GRB 130418A
Techniques: photometric

ASJC Scopus subject areas

Astronomy and Astrophysics
Space and Planetary Science

Access to Document

10.1051/0004-6361/202554710Licence: CC BY

Cite this

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title = "The GROND gamma-ray burst sample: II. Fireball parameters for four gamma-ray burst afterglows",

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.",

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AU - Schady, P.

AU - Van Eerten, H.

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Y1 - 2025/9/30

N2 - 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.

AB - 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.

KW - Gamma-ray burst: general

KW - Gamma-ray burst: individual: GRB 100418A

KW - Gamma-ray burst: individual: GRB 110715A

KW - Gamma-ray burst: individual: GRB 121024A

KW - Gamma-ray burst: individual: GRB 130418A

KW - Techniques: photometric

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ER -

The GROND gamma-ray burst sample: II. Fireball parameters for four gamma-ray burst afterglows

Abstract

Acknowledgements

Funding

Keywords

ASJC Scopus subject areas

Access to Document

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