Physical Conditions That Led to the Detection of the Pair Annihilation Line in the Brightest-of-all-time GRB 221009A

The brightest-of-all-time GRB 221009A shows evidence for a narrow evolving MeV emission line. Here, we show that this line is due to pair annihilation in the prompt emission region and that its temporal evolution is naturally explained as a high-latitude emission (emission from higher angles from the line of sight) after prompt emission is over. We consider both the high and low optical depth for pair production regimes and find acceptable solutions, with the gamma-ray burst (GRB) Lorentz factor Γ ≈ 600 and the emission radius r ≳ 1016.5 cm. We discuss the conditions for the appearance of such a line and show that a unique combination of high luminosity and Lorentz factor that is in a fairly narrow range are required for the line detection. This explains why such an annihilation line is rarely observed in GRBs.