Spectral Shapes of Pair Annihilation Line Emission in Magnetar Giant Flares

TL;DR

This paper models the spectral shape of gamma-ray line emission from electron-positron annihilation in magnetar giant flares, showing that relativistic effects produce a broad power-law feature detectable by current and future gamma-ray instruments.

Contribution

It introduces an analytic model and Monte Carlo simulations to describe the spectral shape and detectability of pair annihilation lines in magnetar giant flares, considering baryon loading effects.

Findings

Relativistic beaming broadens the annihilation line into a power-law with photon index -1.

Baryon-poor fireballs are more detectable in MeV gamma rays.

Current and upcoming gamma-ray instruments can observe these spectral features.

Abstract

We investigate the gamma-ray spectrum in the MeV range arising from electron-positron pair annihilation in fireballs associated with magnetar giant flares (MGFs), motivated by the recent observation of a MeV gamma-ray line feature in a bright gamma-ray burst, GRB~221009A. We develop an analytic model of line emission, demonstrating that relativistic beaming results in a broadened, power-law spectral feature with photon index -1. We then perform Monte Carlo radiative transfer simulations incorporating electron-positron pair production, annihilation, and Compton scattering. The dependence of the emergent spectrum on the baryon loading is also examined, showing that a baryon-poor fireball is more favorable for the detection of MeV gamma rays. We further assess the detectability of the line component. The simulation results indicate that a power-law MeV component from the initial spike of a Galactic MGFs could be observed with current instruments, such as Fermi/GBM, and will be well within the reach of upcoming MeV gamma-ray satellites, which are expected to detect O(100) photons from such events.