Issue |
A&A
Volume 693, January 2025
|
|
---|---|---|
Article Number | A320 | |
Number of page(s) | 14 | |
Section | Astrophysical processes | |
DOI | https://doi.org/10.1051/0004-6361/202451369 | |
Published online | 29 January 2025 |
Gamma-ray burst prompt emission from the synchrotron radiation of relativistic electrons in a rapidly decaying magnetic field
1
Sorbonne Université, CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 Bis bd Arago, 75014 Paris, France
2
Institut Universitaire de France, Ministère de l’Enseignement Supérieur et de la Recherche, 1 rue Descartes, 75231 Paris Cedex F-05, France
3
Faculty of Electrical Engineering and Computing, University of Zagreb, 10000 Zagreb, Croatia
4
Laboratoire Univers et Théories, Observatoire de Paris, Université PSL, CNRS, F-92190 Meudon, France
⋆ Corresponding authors; daigne@iap.fr; Zeljka.Bosnjak@fer.hr
Received:
3
July
2024
Accepted:
11
November
2024
Context. Synchrotron radiation from accelerated electrons above the photosphere of a relativistic ejecta is a natural candidate for the dominant radiative process for the prompt gamma-ray burst emission. There is, however, a tension between the predicted low-energy spectral index, α = −3/2, in the fast cooling regime and observations.
Aims. Radiating electrons have time to travel away from their acceleration site and may experience an evolving magnetic field. We study the impact of a decaying field on the synchrotron spectrum.
Methods. We computed the radiation from electrons in a decaying magnetic field, including adiabatic cooling, synchrotron radiation, inverse Compton scatterings, and pair production. We explored the physical conditions in the co-moving frame of the emission region and focused on the fast cooling regime where the radiative timescale of electrons with a Lorentz factor Γm responsible for the peak of the emission, tsyn(Γm), is much shorter than the dynamical timescale tdyn.
Results. We find that the effect of the magnetic field decay depends on its characteristic timescale tB: (i) for a slow decay with tB ≳ 10 tsyn(Γm), the effect is very weak and the spectral shape is mostly determined by the impact of the inverse Compton scatterings on the electron cooling, leading to −3/2 ≤ α ≤ −1, and (ii) for a fast decay with 0.1 tsyn(Γm)≲tB ≲ 10 tsyn(Γm), the magnetic field decay has a strong impact, leading naturally to the synchrotron marginally fast cooling regime, where α tends to −2/3, while the radiative efficiency remains high. The high-energy inverse Compton component is enhanced in this regime. (iii) For an even faster decay, the whole electron population is in the slow cooling regime.
Conclusions. We conclude that efficient synchrotron radiation in a rapidly decaying magnetic field can reproduce low-energy photon indices ranging from α = −3/2 to −2/3, which is in agreement with the measured value in the majority of gamma-ray burst spectra.
Key words: radiation mechanisms: non-thermal / methods: numerical / gamma-ray burst: general
© The Authors 2025
Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
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