HESS observations of y-ray bursts in 2003-2007

Abstract

Aims. Very-high-energy (VHE; $\ga$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. Methods. GRBs have been one of the prime targets for the HESS 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. Results from observations of 22 GRB positions are presented. 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