[Paper Review] Searching for TeV emission from GRBs: the status of the H.E.S.S. GRB programme

D. Lennarz, P. M. Chadwick+4 more|arXiv (Cornell University)|Jul 26, 2013

Gamma-ray bursts and supernovae1 references3 citations

TL;DR

This paper reviews the status of the H.E.S.S. gamma-ray observatory's programme to detect very-high-energy (VHE) emission from gamma-ray bursts (GRBs), highlighting the enhanced sensitivity from the addition of H.E.S.S. II—the world's largest IACT—featuring a 600 m² mirror, lower energy threshold (tens of GeV), and faster repointing. Despite no significant VHE emission detected from GRB 100621A, the study establishes critical upper limits on potential high-energy spectral components, demonstrating H.E.S.S. II's readiness for future GRB detections.

ABSTRACT

H.E.S.S. is an array of five Imaging Atmospheric Cherenkov Telescopes (IACTs) located 1800 m above sea level in the Khomas Highland of Namibia and is sensitive to very-high-energy (VHE) gamma rays between tens of GeV to tens of TeV. The very-high background rejection capabilities of IACTs provide excellent sensitivity of H.E.S.S. to GRBs. In this contribution the status of the H.E.S.S. GRB programme, already started in 2003, is reviewed. A highlight is the recent addition of the fifth telescope, which is the world's largest IACT. Its 600 square metre mirror lowers the energy threshold to tens of GeV and provides an effective area that is ten thousands of times larger than Fermi-LAT at these energies. The higher performance drive system will reduce the response time to a GRB alert, which will significantly enhance the chances of a H.E.S.S. GRB detection. Recent results on selected GRBs will be shown.

Motivation & Objective

• To assess the status and capabilities of the H.E.S.S. GRB programme in detecting very-high-energy (VHE) gamma-ray emission from GRBs.
• To evaluate the impact of H.E.S.S. II’s upgrades—larger mirror, lower energy threshold, faster repointing—on improving detection prospects.
• To analyze observational data from selected GRBs, particularly GRB 100621A, to set upper limits on VHE emission and test spectral models.
• To understand the implications of VHE emission for GRB emission mechanisms and extragalactic background light (EBL) absorption.

Proposed method

• Utilizes the H.E.S.S. array of five Imaging Atmospheric Cherenkov Telescopes (IACTs) located in Namibia, sensitive to VHE gamma rays from 30 GeV to tens of TeV.
• Employs stereoscopic image analysis using at least two telescopes to reconstruct air showers and improve background rejection.
• Applies the reflected background method for point-source analysis to search for excess gamma-ray events above background.
• Uses automated alert systems via the GCN to respond rapidly to GRB triggers, with H.E.S.S. II reducing response time significantly.
• Performs spectral extrapolation of prompt GRB emission, correcting for EBL absorption, to compare with H.E.S.S. upper limits.
• Analyzes data in time bins (e.g., first 300 s post-trigger) to search for short-timescale VHE emission.

Experimental results

Research questions

• RQ1 Can H.E.S.S. detect VHE gamma-ray emission from GRBs, particularly those with high-luminosity prompt emission and extended high-energy emission seen by Fermi-LAT?
• RQ2 How do the improved capabilities of H.E.S.S. II—lower energy threshold and faster repointing—enhance the probability of detecting VHE GRB emission?
• RQ3 What constraints do H.E.S.S. upper limits place on the existence of hard power-law components in the high-energy spectra of GRBs like GRB 100621A?
• RQ4 To what extent does extragalactic background light (EBL) absorption affect the detectability of VHE GRB emission, and how can upper limits inform EBL models?
• RQ5 Why were several GRB alerts not observed by H.E.S.S., and what operational and technical challenges affect timely follow-up?

Key findings

• H.E.S.S. did not detect significant VHE gamma-ray emission from GRB 100621A, the closest and brightest prompt burst observed so far by H.E.S.S. with a redshift of z=0.542.
• The H.E.S.S. upper limit on VHE emission from GRB 100621A rules out a hard power-law spectral component extending to tens of GeV, assuming prompt emission is not delayed.
• The addition of H.E.S.S. II lowered the energy threshold to tens of GeV and increased the effective area by a factor of 10,000 compared to Fermi-LAT at these energies.
• The faster repointing system of H.E.S.S. II reduced the response time to GRB alerts, significantly improving the chances of detecting prompt VHE emission.
• Despite favorable conditions, observational opportunities were lost due to weather, technical issues, or delayed redshift information, highlighting operational challenges.
• The study demonstrates that H.E.S.S. II is now capable of probing the VHE emission from GRBs, especially those with extended or delayed high-energy emission as seen by Fermi-LAT.