Probing gamma-ray bursts observed at very high energies through their afterglow

Ersilia Guarini, Irene Tamborra, Damien Bégué, Annika Rudolph

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A growing number of gamma-ray burst (GRB) afterglows is observed at very-high energies (VHE, ≳ 100 GeV). Yet, our understanding of the mechanism powering the VHE emission remains baffling. We make use of multiwavelength observations of the afterglow of GRB 180720B, GRB 190114C, and GRB 221009A to investigate whether the bursts exhibiting VHE emission share common features. We assume the standard afterglow model and microphysical parameters consistent with a synchrotron self-Compton (SSC) scenario for the VHE radiation. By requiring that the blastwave should be transparent to γ -γ pair production at the time of observation of the VHE photons and relying on typical prompt emission efficiencies and data in the radio, optical, and X-ray bands, we infer for those bursts that the initial energy of the blastwave is E͂k,iso ≳ O(1054) erg and the circumburst density is n0 ≲ O(10−1) cm−3 for a constant circumburst profile [or A ≲ O(10−1) cm−1 for a wind scenario]. Our findings thus suggest that these VHE bursts might be hosted in low-density environments, if the SSC radiation is responsible for the VHE emission. While these trends are based on a small number of bursts, the Cherenkov Telescope Array has the potential to provide crucial insight in this context by detecting a larger sample of VHE GRBs. In addition, due to the very poor statistics, the non-observation of high-energy neutrinos cannot constrain the properties of these bursts efficiently, unless additional VHE GRBs should be detected at distances closer than 15 Mpc when IceCube-Gen2 radio will be operational.

Original languageEnglish
Pages (from-to)149-162
Number of pages14
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
StatePublished - 1 Jul 2023

Bibliographical note

Publisher Copyright:
© 2023 The Author(s) Published by Oxford University Press on behalf of Royal Astronomical Society.


We are very grateful to Jochen Greiner for insightful discussions. This project has received funding from the Villum Foundation (Project Number 37358), the Carlsberg Foundation (CF18-0183), the MERAC Foundation, the Deutsche Forschungsgemeinschaft through Sonderforschungsbereich SFB 1258 ‘Neutrinos and Dark Matter in Astro- and Particle Physics’ (NDM), and the European Research Council via the ERC Consolidator Grant Number 773062 (acronym O. M. J.).

FundersFunder number
Villum Fonden37358
European Research Council773062
Deutsche ForschungsgemeinschaftSFB 1258
Fondation Merac


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    • acceleration of particles
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