Physical conditions that lead to the detection of the pair annihilation line in the BOAT GRB221009A

TL;DR

This paper explains the detection of a narrow pair annihilation line in the brightest GRB by modeling pair annihilation in the prompt emission region and analyzing the conditions for its observability.

Contribution

It introduces a model for the pair annihilation line in GRB prompt emission, considering high and low optical depth regimes, and identifies specific conditions for its detection.

Findings

Lorentz factor approximately 600

Emission radius greater than 10^{16.5} cm

Rare occurrence explained by narrow luminosity and Lorentz factor range

Abstract

The brightest of all time (BOAT) GRB221009A show evidence for a narrow, evolving MeV emission line. Here, we show that this line can be explained as due to pair annihilation in the prompt emission region, and that its temporal evolution is naturally explained as the 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 GRB Lorentz factor $\Gamma \approx 600$ and the emission radius $r \gtrsim 10^{16.5}$~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.