LST-1 follow-up of the exceptionally bright gamma-ray burst GRB 221009A

Arnau Aguasca-Cabot; Alessandro Carosi; Alice Donini; Susumu Inoue; Yuri Sato; Monica Seglar Arroyo; Kenta Terauchi; Pol Bordas; Marc Ribó

arXiv · 2512.01747

LST-1 follow-up of the exceptionally bright gamma-ray burst GRB 221009A

Arnau Aguasca-Cabot·Alessandro Carosi·Alice Donini·Susumu Inoue·Yuri Sato·Monica Seglar Arroyo·Kenta Terauchi·Pol Bordas·Marc Ribó

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Abstract

On 9 October 2022, the brightest gamma-ray burst (GRB) ever recorded (GRB 221009A) was detected. Its remarkably bright emission, partially due to its close distance to Earth ($z\sim0.15$), makes this GRB a unique event. The outstanding characteristics of GRB 221009A, including the TeV detection by the LHAASO experiment, triggered deep follow-up observations of the source across all wavebands, including very-high-energy gamma rays with the first Large-Sized Telescope (LST-1) of the future Cherenkov Telescope Array Observatory. LST-1 observations started about one day after the onset of the prompt emission, under strong moonlight conditions. This resulted in a hint of a signal with a statistical significance of about 4$\sigma$. The monitoring of this source continued until the end of November 2022. This constitutes the deepest observation campaign performed on a GRB with LST-1. Here we show the results of this follow-up campaign.

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Arnau Aguasca-Cabot, Alessandro Carosi, Alice Donini, Susumu Inoue, Yuri Sato, Monica Seglar Arroyo, Kenta Terauchi, Pol Bordas, Marc Ribó. 2025-12-01. LST-1 follow-up of the exceptionally bright gamma-ray burst GRB 221009A. https://arxiv.org/abs/2512.01747

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