Gamma-ray burst prompt emission spectra at high energies

¹ Gran Sasso Science Institute, Viale F. Crispi 7, I-67100 L’Aquila (AQ), Italy
² INFN – Laboratori Nazionali del Gran Sasso, I-67100 L’Aquila (AQ), Italy
³ INAF – Osservatorio Astronomico d’Abruzzo, Via M. Maggini snc, I-64100 Teramo, Italy

^⋆ Corresponding author: samanta.macera@gssi.it

Received: 17 January 2025
Accepted: 25 May 2025

Abstract

Despite more than fifty years of gamma-ray burst (GRB) observations, several questions regarding the origin of the prompt emission, particularly at high energies, remain unresolved. We present a comprehensive analysis of 35 GRBs observed by Fermi/GBM and Fermi/LAT over the past 15 years, focusing on the nature of high-energy (HE, E > 100 MeV) emission during the prompt emission phase. Our study combines temporal and spectral analyses to investigate the synchrotron origin of the observed emission spanning the energy range from 10 keV to 100 GeV and explore the possible contribution of additional spectral components. Temporal modeling of Fermi/LAT light curves for 12 GRBs in our sample reveals deviations from standard afterglow scenarios during the early phases, suggesting a significant contamination from prompt emission. We find that most GRB spectra align with synchrotron emission extending to GeV energies, with the slope p of the non-thermal electron distribution clustering around p ∼ 2.7, consistently with theoretical predictions. For three GRBs, an additional power law component is required to explain the high-energy emission, but the nature and temporal evolution of this component remain unclear due to the limited quality of Fermi/LAT data. When the power law component is needed, the synchrotron spectrum shows a sharp MeV suppression. It could be explained by the pair loading effects in the early afterglow. These findings emphasize the importance of multi-wavelength observations in unveiling the mechanisms driving early HE prompt emission in GRBs. We briefly discuss the implications of our findings for future very-high-energy (VHE, E > 100 GeV) gamma-ray observatories, such as the Cherenkov Telescope Array, and address the detection prospects of additional non-thermal components in GRB spectra.