GRB 221009A: Observations with LST-1 of CTAO and implications for structured jets in long gamma-ray bursts

TL;DR

This study reports LST-1 observations of GRB 221009A starting at $T_0+1.33$ d, including moonlight data analysis, and detects a possible VHE signal at 4.1$\sigma$ on that night. By comparing the VHE results with structured-jet afterglow models, the authors disfavor models predicting VHE flux above $1\times 10^{-11}$ erg cm$^{-2}$ s$^{-1}$ at $\sim$1 day and show that late-time VHE measurements can help discriminate between inner and outer jet components. The work demonstrates the feasibility of moonlight analyses for IACTs and provides new constraints on jet structure in long GRBs, highlighting the need for broad MWL coverage and early-to-late VHE observations to reduce model degeneracies. The findings have implications for understanding jet formation and propagation in long GRBs and for planning future CTAO investigations.

Abstract

GRB 221009A is the brightest gamma-ray burst (GRB) observed to date. Extensive observations of its afterglow emission across the electromagnetic spectrum were performed, providing the first strong evidence of a jet with a nontrivial angular structure in a long GRB. We carried out an extensive observation campaign in very-high-energy (VHE) gamma rays with the first Large-Sized Telescope (LST-1) of the future Cherenkov Telescope Array Observatory (CTAO), starting on 2022 October 10, about one day after the burst. A dedicated analysis of the GRB 221009A data is performed to account for the different moonlight conditions under which data were recorded. We find an excess of gamma-like events with a statistical significance of 4.1$σ$ during the observations taken 1.33 days after the burst, followed by background-compatible results for the later days. The results are compared with various models of afterglows from structured jets that are consistent with the published multiwavelength data, but entail significant quantitative and qualitative differences in the VHE emission after one day. We disfavor models that imply VHE flux at one day considerably above $10^{-11}$ erg cm$^{-2}$ s$^{-1}$. Our late-time VHE observations can help disentangle the degeneracy among the models and provide valuable new insight into the structure of GRB jets.

Figures (7)

• Figure 1: $\theta^2$ plot for observations at $T_0+1.33$ d. The $\theta^2$ distributions centered at the GRB 221009A position (ON) and the mean background estimation from the three reflected regions (OFF) are displayed as black points and dark orange error bars, respectively. The vertical dashed line indicates the angular size used to compute the detection statistical significance (Li$\&$Ma Sig.) and signal-to-background ratio (S/B). The vertical error bars correspond to 1$\sigma$ statistical errors.
• Figure 2: Intrinsic SED of GRB 221009A corrected for EBL attenuation on 2022 October 10 ($T_0+1.33\,$d; pink), 2022 October 12 ($T_0+3.33\,$d; brown) and between 2022 October 15 and 27 (all dark, $T_0+[6.30,17.32]\,$d; olive), respectively. For the latter SED, the diamond and square olive empty markers show the effect of increasing and reducing by $0.5\%$ the normalization of the background, respectively. The vertical error bars correspond to 1$\sigma$ statistical errors and ULs are computed at the 95% confidence level.
• Figure 3: MWL intrinsic light curve of GRB 221009A corrected for EBL attenuation versus time since the burst trigger ($T_0$) shifted by $+226$ s (see text). The energy fluxes for the 0.3--5 TeV band with LHAASO LHAASO2023KM2A, HAWC HAWC2022GCN, LST-1 (black filled and empty circles for ULs and the energy flux point, respectively; this work) and H.E.S.S. HESS2023 are compared with the best-fit emission models from Ren2024Zhang2025Zheng2024 in pale gold, red and purple, respectively. For the latter two models, the contributions from the inner and outer jet regions are also shown separately. In addition, the light curve in HE gamma rays with AGILE-GRID tavani2023agile and Fermi-LAT axelsson2024FermiLAT as well as in X-rays with Swift-XRT Williams2023swift are displayed. The vertical error bars correspond to 1$\sigma$ statistical errors and ULs are computed at the 95% confidence level.
• Figure 4: Same as Figure \ref{['fig:theta2plot_Oct10']}, but displaying the $\alpha$ distributions at $T_0+1.33$ d after analysis cuts. The vertical dashed line indicates the $\alpha$ cut value, below which the detection statistical significance (Li$\&$Ma Sig.) is computed. The vertical error bars correspond to 1$\sigma$ statistical errors.
• Figure 5: Comparison of SEDs between source-independent (black) and source-dependent (blue) approaches using the datasets on 2022 October 10 ($T_0+1.33\,$d; left), 2022 October 12 ($T_0+3.33\,$d; middle) and between 2022 October 15 and 27 (all dark, $T_0+[6.30,17.32]\,$d; right). ULs are computed at 95% confidence level when the test statistic ($TS$) is below 4. Error uncertainties correspond to 1$\sigma$ statistical errors.