Constraining the jet composition of GRB 221009A with the prompt TeV emission limit

TL;DR

This study uses LHAASO observations and simulations to analyze the TeV and MeV emission ratio in GRB 221009A, concluding the jet is likely Poynting-flux-dominated due to suppressed SSC emission.

Contribution

It introduces a simulation-based analysis of gamma-ray absorption and emission mechanisms to constrain the jet composition of GRB 221009A.

Findings

Gamma-gamma absorption produces a power-law spectrum, not an exponential cutoff.

The flux ratio limit implies a low SSC to synchrotron emission ratio.

The jet is likely Poynting-flux-dominated.

Abstract

Recent LHAASO observations of the prompt emission phase of the brightest-of-all-time GRB 221009A imposes a stringent limit on the flux ratio between the TeV and MeV emissions, $F_{\rm TeV}/F_{\rm MeV}\le 2\times10^{-5}$, during the period $220 \operatorname{-}230\, {\rm s}$ after the trigger. bf This period covers the peak of the main MeV burst and is just before the TeV afterglow emerges. Within the framework of internal shocks, we study the internal $\gamma\gamma$ absorption in GRB 221009A by generating a set of synthetic bursts in a simulation that reproduces the observed feature of GRB 221009A. We find that the $\gamma\gamma$ absorption does not lead to an exponential cutoff, but rather a power-law spectrum, consistent with previous works. We further find that the attenuation due to $\gamma\gamma$ absorption alone cannot explain the flux limit ratio of GRB 221009A, suggesting a low ratio between synchrotron self-Compton (SSC) and synchrotron emission outputs. This requires the magnetic field energy density to be much larger than the synchrotron photon energy density so that the SSC flux is greatly suppressed. This indicates that the jet composition of GRB 221009A is likely Poynting-flux-dominated.