Hybrid Emission Modeling of GRB 221009A: Shedding Light on TeV Emission Origins in Long-GRBs

TL;DR

This paper models the TeV afterglow of the energetic GRB 221009A using a hybrid proton-synchrotron and electron-synchrotron approach, constraining parameters to explain its high-energy emission and suggesting proton-synchrotron as a key mechanism.

Contribution

It introduces a hybrid emission model for GRB afterglows, specifically applying it to GRB 221009A, and constrains physical parameters to support proton-synchrotron as a dominant TeV emission process.

Findings

TeV emission requires large kinetic energy (~10^{54} erg) and dense circumburst medium.

Proton-synchrotron dominates over SSC in producing TeV photons under the model conditions.

Small electron acceleration fraction (~10^{-3}) and near-equipartition magnetic fields are necessary.

Abstract

Observations of long duration gamma-ray bursts (GRBs) with TeV emission during their afterglow have been on the rise. Recently, GRB 221009A, the most energetic GRB ever observed, was detected by the {LHAASO} experiment in the energy band 0.2 - 7 TeV. Here, we interpret its afterglow in the context of a hybrid model in which the TeV spectral component is explained by the proton-synchrotron process while the low energy emission from optical to X-ray is due to synchrotron radiation from electrons. We constrained the model parameters using the observed optical, X-ray and TeV data. By comparing the parameters of this burst and of GRB 190114C, we deduce that the VHE emission at energies $\geq$ 1 TeV in the GRB afterglow requires large explosion kinetic energy, $E \gtrsim 10^{54}$~erg and a reasonable circumburst density, $n\gtrsim 10$~cm$^{-3}$. This results in a small injection fractions of particles accelerated to a power-law, $\sim 10^{-2}$. {A significant fraction of shock energy must be allocated to a near equipartition magnetic field, $\epsilon_B \sim 10^{-1}$, while electrons should only carry a small fraction of this energy, $\epsilon_e \sim 10^{-3}$. Under these conditions required for a proton synchrotron model, namely $\epsilon_B \gg \epsilon_e$, the SSC component is substantially sub-dominant over proton-synchrotron as a source of TeV photons.} These results lead us to suggest that proton-synchrotron process is a strong contender for the radiative mechanisms explaining GRB afterglows in the TeV band.