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Spectral Shapes of Pair Annihilation Line Emission in Magnetar Giant Flares

Tomoki Wada, Shigeo S. Kimura

TL;DR

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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.

Spectral Shapes of Pair Annihilation Line Emission in Magnetar Giant Flares

TL;DR

...

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.
Paper Structure (3 sections, 4 equations, 2 figures)

This paper contains 3 sections, 4 equations, 2 figures.

Table of Contents

  1. Introduction
  2. Analytic Model of Spectral Shape
  3. Numerical Calculation

Figures (2)

  • Figure 1: Spectra of the observed line emission from fireballs with different specific enthalpies (solid lines), the primary blackbody radiation (black dashed line), and analytic model derived in Section \ref{['sec:ana']} (gray dotted line). The adopted parameters are shown in Section \ref{['sec:num']}. The spectra originally characterized by Equation (\ref{['eq:specflux']}) is modified during propagation due to Compton scattering (see Section \ref{['sec:ana']}).
  • Figure 2: $\eta$-dependence of the line luminosity $L_{\rm line}$ normalized by the blackbody luminosity $L_{\rm BB}$. The color scheme follows that of Figure \ref{['fig:spec']} for ease of comparison.