On the External Inverse Compton Scattering off the Prompt Emission in GRB 221009A

Abstract The temporal evolution of the TeV emission of GRB 221009A displays a smooth transition from an initial rapid rise to a slower rise and eventually a decay phase. The smooth light curve suggests an afterglow origin of the TeV emission. The temporal overlap between the prompt keV–MeV emission and the early TeV afterglow indicates that external inverse Compton (EIC) scattering between the prompt keV–MeV photons and the afterglow electrons is inevitable. Since the energy density of the prompt emission is much higher than that of the afterglow during the early phase, the EIC process dominates the cooling of afterglow electrons. The EIC scattering rate is influenced by the anisotropy of the seed photon field, which depends on the radius of the internal dissipation ( R dis ), where the prompt emission is produced, and that of the external shock ( R ext ), where the afterglow emission is produced. We investigate the EIC process for different values of R dis / R ext and find that the EIC scattering rate can differ by a factor of ∼2 accordingly. For GRB 221009A, the EIC emission dominates during the early rising phase of the TeV afterglow. It then transitions to a phase dominated by the synchrotron self-Compton emission as the intensity of the prompt emission decreases. Additionally, we investigate the effect of γγ absorption in the TeV afterglow by prompt MeV photons and find it insufficient to explain the early rapid rise in the TeV afterglow.