The contemporaneous phase of GRB afterglows – application to GRB 221009A

The TeV observations of GRB 221009A provided us with a unique opportunity to analyse the contemporaneous phase in which both prompt and afterglow emissions are seen simultaneously. To describe this initial phase of gamma-ray burst afterglows, we suggest a model for a blast wave with an intermittent energy supply. We treat the blast wave as a two-element structure. The central engine supplies energy to the inner part (shocked ejecta material) via the reverse shock. As the shocked ejecta material expands, its internal energy is transferred to the shocked external matter. We take into account the inertia of the shocked external material so that the pressure difference across this region determines the derivative of the blast wave’s Lorentz factor. Applied to GRB 221009A, the model yields a very good fit to the observations of the entire TeV light curve except for three regions where there are excesses in the data with respect to the model. Those are well correlated with the three largest episodes of the prompt activity and thus, we interpret them as the reverse shock emission. Our best-fitting solution for GRB 221009A is an extremely narrow jet with an opening angle θ_j ≈ 0.07^◦ (500/Γ₀) propagating into a wind-like external medium. This extremely narrow angle is consistent with the huge isotropic equivalent energy of this burst, and its inverse jet break explains the very rapid rise of the afterglow. Such inverse jet break occurs in an accelerating blast wave when the relativistic beaming becomes narrower than the jet’s opening angle. Interestingly, photon–photon annihilation does not play a decisive role in the best-fitting model.