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

TL;DR

This study reports the LST-1 follow-up of the exceptionally bright GRB 221009A, initiating observations at $T_0+1.33$ d under moonlight and employing moonlight-tolerant analysis alongside a standard pipeline. A preliminary $\sim4\sigma$ hint is found on the first night, with subsequent data consistent with background; the work provides deep upper limits on the SED down to $\sim50$–$200$ GeV after correcting for EBL attenuation with $z=0.1505$ and $z$-dependent models. The analysis demonstrates LST-1's capability to operate under challenging NSB conditions and informs the design of CTAO follow-up strategies and high-energy emission modeling for nearby, bright GRBs. Final, comprehensive results are presented in a companion paper (2025ApJ...988L..42A).

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$σ$. 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.

Figures (2)

• Figure 1: $\theta^2$ plots. From top to bottom, the angular distribution of events centred on GRB 221009A (ON) is compared with the mean angular distribution of events obtained at three OFF regions (OFF) using data on Oct. 10, Oct. 12 and Oct. 15--27. The detection statistical significance (Li&Ma Sig.) and signal-to-background ratio (S/B) is shown for events within the $\theta^2$ cut. Vertical errors bars are $1\sigma$ statistical uncertainties.
• Figure 2: SEDs corrected for EBL attenuation. From top to bottom, the constraints on the SEDs using data on Oct. 10, Oct. 12 and Oct. 15--27 are shown. For each plot, the tests of increasing and reducing by $0.5\%$ the background normalisation are shown using dotted-dashed and dotted error bars, respectively. This effect is negligible for the analysis of data on Oct. 10 and Oct. 12. Vertical errors bars are $1\sigma$ statistical uncertainties. Upper limits are computed at 95% confidence level.