Gamma-gamma absorption in gamma-ray burst environments

Abstract

Gamma-ray bursts (GRBs) are the most violent explosions in the universe, seen primarily as bright, short flashes of g-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 g-rays by satellite-borne g-ray telescopes at MeV – GeV energies, but could potentially also emit g-rays at even higher energies (Very-High-Energy [VHE] g-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 gg absorption for VHE g-rays from GRBs. We find that for the likely properties of OB associations around GRB progenitors, gg absorption is expected to be negligible