H.E.S.S. Follow-up Observations of GRB 221009A

Abstract

GRB 221009A is the brightest gamma-ray burst (GRB) ever detected. To probe the very-high-energy (VHE; >100 GeV) emission, the High Energy Stereoscopic System (H.E.S.S.) began observations 53 hr after the triggering event, when the brightness of the moonlight no longer precluded observations. We derive differential and integral upper limits using H.E.S.S. data from the third, fourth, and ninth nights after the initial GRB detection, after applying atmospheric corrections. The combined observations yield an integral energy flux upper limit of Φ UL 95 % = 9.7 × 10 − 12 erg cm − 2 s − 1 above E thr = 650 GeV. The constraints derived from the H.E.S.S. observations complement the available multiwavelength data. The radio to X-ray data are consistent with synchrotron emission from a single electron population, with the peak in the spectral energy distribution occurring above the X-ray band. Compared to the VHE-bright GRB 190829A, the upper limits for GRB 221009A imply a smaller gamma-ray to X-ray flux ratio in the afterglow. Even in the absence of a detection, the H.E.S.S. upper limits thus contribute to the multiwavelength picture of GRB 221009A, effectively ruling out an IC-dominated scenario.

A Photohadronic Interpretation of H.E.S.S. Afterglow Observations of GRB 221009A

“…The estimated g F int for each δ is smaller than the upper limits of H.E.S.S. (Aharonian et al 2023).…”

“…The H.E.S.S. collaboration started observing GRB 221009A with its 12 m telescopes starting 53 hr after the triggering event when the observing conditions were favorable (Aharonian et al 2023). On 2022 October 11, the H.E.S.S.…”

“…Due to poor atmospheric conditions and high aerosol content in the atmosphere (Formenti et al 2019), the only nights it could observe the source were 2022 October 11, 12, and 17 (Nights 3, 4, and 9, after the triggering event, respectively). Additional observations undertaken on other nights were excluded from the analysis due to worsening of the atmospheric conditions (Aharonian et al 2023). Unfortunately, H.E.S.S.…”

“…Instead, the collaboration computed upper limits to constrain the VHE emission from GRB 221009A. In the energy range 0.65 TeV E γ 10 TeV, the computed integrated flux upper limit at 95% confidence level (CL) for Night 3 is 4.06 × 10 −11 erg cm −2 s −1 and for all nights combined it is 9.7 × 10 −12 erg cm −2 s −1 (Aharonian et al 2023).…”

The Case of the Missing Very High-energy Gamma-Ray Bursts: A Retrospective Study of Swift Gamma-Ray Bursts with Imaging Atmospheric Cherenkov Telescopes

“…However, they unfortunately fell outside IACT observation conditions. GRB 221009A at z = 0.151 (Castro-Tirado et al 2022;de Ugarte Postigo et al 2022;Izzo et al 2022) is the brightest GRB of all time and it occurred during the full moon, preventing IACTs from acquiring data until a few days later with no significant detection (Aharonian et al 2023). This suggests that the relation between the X-rays and the gamma rays might not hold for late times and for all cases.…”