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Recent LHAASO observations of the prompt emission phase of thebrightest-of-all-time GRB 221009A imposes a stringent limit on the flux ratiobetween 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. bfThis period covers the peak of the main MeV burst and is just before the TeVafterglow emerges. Within the framework of internal shocks, we study theinternal $\gamma\gamma$ absorption in GRB 221009A by generating a set ofsynthetic bursts in a simulation that reproduces the observed feature of GRB221009A. We find that the $\gamma\gamma$ absorption does not lead to anexponential cutoff, but rather a power-law spectrum, consistent with previousworks. We further find that the attenuation due to $\gamma\gamma$ absorptionalone cannot explain the flux limit ratio of GRB 221009A, suggesting a lowratio between synchrotron self-Compton (SSC) and synchrotron emission outputs.This requires the magnetic field energy density to be much larger than thesynchrotron photon energy density so that the SSC flux is greatly suppressed.This indicates that the jet composition of GRB 221009A is likelyPoynting-flux-dominated.

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