H.E.S.S. observations of gamma-ray bursts in 2003-2007

TL;DR

This study reports on H.E.S.S. telescope observations of 32 gamma-ray bursts between 2003 and 2007, finding no evidence of very-high-energy gamma-ray emission and setting upper limits on their flux.

Contribution

First comprehensive search for VHE gamma-ray counterparts of GRBs using H.E.S.S. over four years, establishing upper limits and analyzing absorption effects.

Findings

No VHE gamma-ray signals detected from individual GRBs.

Upper limits set on VHE flux for all observed GRBs.

Absorption corrections applied for GRBs with known redshifts.

Abstract

Very-high-energy (VHE; >~100 GeV) gamma-rays are expected from gamma-ray bursts (GRBs) in some scenarios. Exploring this photon energy regime is necessary for understanding the energetics and properties of GRBs. GRBs have been one of the prime targets for the H.E.S.S. experiment, which makes use of four Imaging Atmospheric Cherenkov Telescopes (IACTs) to detect VHE gamma-rays. Dedicated observations of 32 GRB positions were made in the years 2003-2007 and a search for VHE gamma-ray counterparts of these GRBs was made. Depending on the visibility and observing conditions, the observations mostly start minutes to hours after the burst and typically last two hours. Results from observations of 22 GRB positions are presented and evidence of a VHE signal was found neither in observations of any individual GRBs, nor from stacking data from subsets of GRBs with higher expected VHE flux according to a model-independent ranking scheme. Upper limits for the VHE gamma-ray flux from the GRB positions were derived. For those GRBs with measured redshifts, differential upper limits at the energy threshold after correcting for absorption due to extra-galactic background light are also presented.