Letter to the Editor

Ultrahigh-energy cosmic-ray signature in GRB 221009A

¹ Center for Gravitational Physics and Quantum Information, Yukawa Institute for Theoretical Physics, Kyoto University, Kyoto 606-8502, Japan
e-mail: saikat.das@yukawa.kyoto-u.ac.jp
² Centre for Astro-Particle Physics (CAPP) and Department of Physics, University of Johannesburg, PO Box 524 Auckland Park 2006, South Africa
e-mail: srazzaque@uj.ac.za
³ Department of Physics, The George Washington University, Washington, DC 20052, USA
⁴ National Institute for Theoretical and Computational Sciences (NITheCS), Private Bag X1, Matieland, South Africa

Received: 4 November 2022
Accepted: 18 January 2023

Abstract

The brightest long gamma-ray burst (GRB) detected so far by the Swift-BAT and Fermi-GBM telescopes, GRB 221009A, provides an unprecedented opportunity for understanding the high-energy processes in extreme transient phenomena. We find that the conventional leptonic models for the afterglow emission from this source, synchrotron and synchrotron-self-Compton, have difficulties explaining the observation of ≳10 TeV γ rays (as high as 18 TeV) by the LHAASO detector. We modeled the γ-ray spectrum estimated in the energy range 0.1–1 GeV by the Fermi-LAT detector. The flux predicted by our leptonic models is severely attenuated at > 1 TeV due to γγ pair production with extragalactic background light, and hence an additional component is required at ≳10 TeV. Ultrahigh-energy cosmic rays can be accelerated in the GRB blast wave, and their propagation induces an electromagnetic cascade in the extragalactic medium. The line-of-sight component of this flux can explain the emission at ≳10 TeV detected by LHAASO, which requires a fraction of the GRB blast wave energy to be in ultrahigh-energy cosmic rays. This could be an indication of ultrahigh-energy cosmic-ray acceleration in GRBs.