2024ApJ...961L...6G

GRB 221009A produced the highest flux of gigaelectronvolt-teraelectronvolt (GeV-TeV) photons ever observed, allowing the construction of a detailed TeV light curve. We focus on explaining the noticeable dip in the light curve around 2-5 s after the onset of TeV emission. We propose that megaelectronvolt (MeV) photons from the prompt emission annihilate with TeV photons from the afterglow, producing an optical depth that obscures the TeV emission during this period. We develop a two-zone model accounting for the angles of MeV photons that can successfully reproduce the time delay between MeV and TeV photons, the peak optical depth over 3, and the rapid decline in optical depth. Our model supports MeV-TeV annihilation as the cause of the dip and provides reasonable constraints on the emission region parameters.