Early $\mathrm{TeV}$ photons of GRB 221009A were absorbed by the prompt MeV photons

Abstract

Gamma-ray burst (GRB) 221009A produced the highest flux of gigaelectronvolt-teraelectronvolt ($\mathrm{GeV}-\mathrm{TeV}$) photons ever observed, allowing the construction of a detailed $\mathrm{TeV}$ light curve. We focus on explaining the noticeable dip in the light curve around $2$-$5\mathrm{s}$ after the onset of $\mathrm{TeV}$ emission. We propose that Megaelectronvolt (MeV) photons from the prompt emission annihilate with $\mathrm{TeV}$ photons from the afterglow, producing an optical depth that obscures the $\mathrm{TeV}$ 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 $\mathrm{TeV}$ photons, the peak optical depth over 3, and the rapid decline in optical depth. Our model supports $\mathrm{MeV}-\mathrm{TeV}$ annihilation as the cause of the dip and provides reasonable constraints on the emission region parameters.

ApJL under review