MeV absorption in gamma-ray bursts as a probe of their progenitor winds

TL;DR

This paper explores how MeV photon absorption caused by electron-positron pair creation in the environment surrounding gamma-ray bursts can influence their observed spectra, providing insights into the properties of progenitor star winds.

Contribution

It introduces a model for how back-scattered photons modify GRB spectra through gamma-gamma absorption, linking spectral features to progenitor wind characteristics.

Findings

Spectral attenuation structures depend on the low-energy spectral index.

Pronounced saddle-shaped absorption features occur for spectra with alpha > -1.

External MeV absorption may explain observed spectral curvature in bright GRBs.

Abstract

A small fraction of X-ray photons from gamma-ray bursts (GRBs), after escaping the relativistic jet, are scattered by electrons in the circumburst medium. Subsequent photon-photon absorption between the incoming MeV $\gamma$-rays and the back-scattered X-rays generate electron-positron pairs, enriching the surrounding medium with leptons. We investigate how these back-scattered photons modify the prompt GRB spectrum through $\gamma-\gamma$ absorption. In a dense and pair-loaded wind environment, the emerging spectra exhibit a broad attenuation structure, whose morphology is sensitive to the low-energy spectral index $\alpha$. In particular, spectra with $\alpha > -1$ develop a pronounced, saddle-shaped absorption between 1 and 100 MeV (rest frame). Such external MeV absorption could account for the spectral curvature seen in some bright GRBs, and may point to enhanced mass loss from their progenitor stars, consistent with early observations of core-collapse supernovae.