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

TL;DR

This paper reports on the deep follow-up observations of the exceptionally bright gamma-ray burst GRB 221009A using the LST-1 telescope, including a potential 4σ signal detection, providing valuable insights into such rare cosmic events.

Contribution

First LST-1 follow-up of a very bright GRB, demonstrating capabilities under moonlight and providing the deepest observation campaign for a GRB with this telescope.

Findings

Possible 4σ signal detection by LST-1

Deepest GRB observation campaign with LST-1 to date

Multi-wavelength follow-up of a unique, bright GRB

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.