Search for TeV Gamma-ray Emission from GRB 100621A, an extremely bright GRB in X-rays, with H.E.S.S

TL;DR

This study observed the brightest X-ray GRB 100621A with H.E.S.S. to detect very-high-energy gamma rays, but found no emission, providing upper limits that inform models of GRB afterglow emission mechanisms.

Contribution

First H.E.S.S. observation of GRB 100621A, setting upper limits on VHE gamma-ray emission and constraining leptonic afterglow models compared to X-ray flux.

Findings

No VHE gamma-ray emission detected from GRB 100621A.

Established upper limits on VHE flux above 380 GeV.

Constrained the ratio of X-ray to VHE flux to be greater than 0.4.

Abstract

The long gamma-ray burst (GRB) 100621A, at the time the brightest X-ray transient ever detected by Swift-XRT in the $0.3\textrm{--}10$ keV range, has been observed with the H.E.S.S. imaging air Cherenkov telescope array, sensitive to gamma radiation in the very-high-energy (VHE, $>100$ GeV) regime. Due to its relatively small redshift of $z\sim0.5$, the favourable position in the southern sky and the relatively short follow-up time ($<700 \rm{s}$ after the satellite trigger) of the H.E.S.S. observations, this GRB could be within the sensitivity reach of the H.E.S.S. instrument. The analysis of the H.E.S.S. data shows no indication of emission and yields an integral flux upper limit above $\sim$380 GeV of $4.2\times10^{-12} \rm cm^{-2}s^{-1}$ (95 % confidence level), assuming a simple Band function extension model. A comparison to a spectral-temporal model, normalised to the prompt flux at sub-MeV energies, constraints the existence of a temporally extended and strong additional hard power law, as has been observed in the other bright X-ray GRB 130427A. A comparison between the H.E.S.S. upper limit and the contemporaneous energy output in X-rays constrains the ratio between the X-ray and VHE gamma-ray fluxes to be greater than 0.4. This value is an important quantity for modelling the afterglow and can constrain leptonic emission scenarios, where leptons are responsible for the X-ray emission and might produce VHE gamma rays.