Evidence of Hadronic Emission from the brightest-of-all-time GRB 221009A

TL;DR

This paper presents the first observational evidence of hadronic emission in a gamma-ray burst, using comprehensive spectral analysis of GRB 221009A across keV to TeV energies, indicating accelerated hadrons in the early afterglow.

Contribution

It provides the first direct evidence of hadronic emission in a GRB by analyzing multi-wavelength data and modeling combined leptonic and hadronic processes.

Findings

Leptonic models alone cannot explain the observations.

A combined hadronic and leptonic model fits the data well.

TeV light curve contains imprints of prompt MeV emission.

Abstract

Acceleration of hadrons in relativistic shocks has been long expected and invoked to model GRB high-energy photon and neutrino emissions. However, so far there has been no direct observational evidence of hadronic emission from GRBs. The B.O.A.T. ("brightest of all time") gamma-ray burst (GRB) 221009A had extreme energies (with an isotropic energy exceeding $10^{55}$ erg) and was detected in broad-band including the very-high-energy (VHE, $>100\,\rm GeV$) band up to $>10$ TeV. Here we perform a comprehensive spectral analysis of the GRB from keV to TeV energy range and perform detailed spectral and light curve modelings considering both the traditional synchrotron self-Compton process and the electromagnetic (EM) cascade process initiated by hadronic interactions by accelerated cosmic rays in the external shock. We find that the leptonic scenario alone is not adequate to account for the observations, whereas the proposed scenario with the combination of hadronic and leptonic components can well reproduce the multi-wavelength spectra and the light curve. This result reveals the existence of the accelerated hadronic component in the early afterglow of this extreme burst. According to this scenario, the observed TeV light curve should contain imprints of the prompt MeV emission.