Observation of the spectral turnover in the afterglow emission of GRB 221009A

¹ Gran Sasso Science Institute, Viale F. Crispi 7, L’Aquila (AQ) I-67100, Italy
² INFN – Laboratori Nazionali del Gran Sasso L’Aquila (AQ) I-67100, Italy
³ Ioffe Institute, 26 Politekhnicheskaya, St Petersburg 194021, Russia
⁴ Department of Physics, University of Cagliari, SP Monserrato-Sestu, Cagliari, 09042 Cagliari, Italy

^⋆ Corresponding authors: biswajit.banerjee@gssi.it, samanta.macera@gssi.it, gor.oganeysan@gssi.it

Received: 27 March 2025
Accepted: 30 June 2025

Abstract

The afterglow emission of gamma-ray bursts (GRBs) originates from deceleration of the ultra-relativistic matter in the circumburst medium. Observations of afterglow in the high-energy and very high-energy domains provide essential information about the physics of acceleration in relativistic shocks. The lack of sensitivity in the mega-electronvolt gamma rays in the afterglow phase prevents us from constraining the spectral turnover from the X-ray to tera-electronvolt energies. GRB 221009A offers the unique opportunity to characterize spectral components during the prompt and early afterglow phases and probe their origin, including mega-electronvolt observations. Analyzing the multiwave band afterglow emission, we identified two distinct spectral components during the initial 30 minutes. The second spectral component peaks at ∼100 GeV, and the bolometric fluence (10 MeV–10 TeV) is greater than 2 × 10⁻³ erg/cm². Performing broadband spectral modeling, we provide constraints on the magnetic field (∼0.1 G) and the energies of the accelerated electrons in the external relativistic shock.