Camelidae on BOAT: observation of a second spectral component in GRB 221009A

TL;DR

This paper reports the detection of a second spectral component in the brightest gamma-ray burst, GRB 221009A, revealing new insights into its high-energy emission and the physical conditions of the shock environment.

Contribution

It presents the first observation of a distinct VHE spectral component in GRB 221009A, with detailed broadband spectral modeling and physical interpretation.

Findings

Identification of a second spectral component peaking between 10-300 GeV

Bolometric fluence exceeds 2×10^{-3} erg/cm^2

Constraints on magnetic field and electron energies in the shock

Abstract

Observing and understanding the origin of the very-high-energy (VHE) spectral component in gamma-ray bursts (GRBs) has been challenging because of the lack of sensitivity in MeV-GeV observations, so far. The majestic GRB 221009A, known as the brightest of all times (BOAT), offers a unique opportunity to identify spectral components during the prompt and early afterglow phases and probe their origin. Analyzing simultaneous observations spanning from keV to TeV energies, we identified two distinct spectral components during the initial 20 minutes of the burst. The second spectral component peaks between $10-300$ GeV, and the bolometric fluence (10 MeV-10 TeV) is estimated to be greater than 2$\times10^{-3}$ erg/ cm$^{2}$. Performing broad-band spectral modeling, we provide constraints on the magnetic field and the energies of electrons accelerated in the external relativistic shock. We interpret the VHE component as an afterglow emission that is affected by luminous prompt MeV radiation at early times.