# Gamma-Gamma Absorption in Gamma-Ray Burst Environments

## TL;DR

This paper investigates the gamma-gamma absorption of very-high-energy gamma-rays emitted by gamma-ray bursts within star-forming regions, finding that such absorption is generally negligible in typical OB association environments.

## Contribution

It provides the first detailed estimate of gamma-gamma opacity in GRB environments, showing that VHE gamma-ray absorption is minimal around typical OB associations.

## Key findings

- Gamma-gamma absorption is negligible in typical OB association environments.
- VHE gamma-rays from GRBs are unlikely to be significantly absorbed by local stellar radiation.
- The study helps clarify the prospects for detecting VHE gamma-rays from GRBs.

## Abstract

Gamma-ray bursts (GRBs) are the most violent explosions in the universe, seen primarily as bright, short flashes of gamma-rays. Long GRBs are most likely associated with the violent death of a very massive star. They are thus believed to originate within regions of recent or ongoing star formation with various bright, young stars, for instance, OB associations. GRBs have been detected in gamma-rays by satellite-borne gamma-ray telescopes at MeV - GeV energies, but could potentially also emit gamma-rays at even higher energies (Very-High-Energy [VHE] gamma-rays: E > 100 GeV), as observed by ground-based Cherenkov Telescope facilities, such as the High Energy Stereoscopic System (H.E.S.S) in Namibia. VHE gamma-rays can be absorbed by low energy photons by pair production, and the stars in the vicinity of a GRB provide a dense radiation environment, which could lead to such absorption. We have investigated representative GRB environments and provide an estimate of the opacity to gamma-gamma absorption for VHE gamma-rays from GRBs. We find that for the likely properties of OB associations around GRB progenitors, gamma-gamma absorption is expected to be negligible.