Catching TeV emission from GRB 221009A and alike with LHAASO, LACT and SWGO

Abstract

Gamma-Ray Bursts (GRBs) are the most energetic electromagnetic explosions in the universe. Recently, the Large High Altitude Air Shower Observatory (LHAASO) reported the breakthrough observation of GRB 221009A with gamma-ray energies beyond 13 TeV. This discovery, together with the previous GRB detection well above 100 GeV, confirms the production of very-high-energy (VHE, $\gtrsim 100$ GeV) radiation which might be a common component of all bright GRBs. It is reasonable to expect that bright GRBs are important targets for ground-based gamma-ray experiments. In this work, we estimate the detection rate for current and upcoming ground-based gamma-ray observatories including LHAASO, Large Array of Imaging Atmospheric Cherenkov Telescopes (LACT) and the Southern Wide-field Gamma-ray Observatory (SWGO) under two emission models with GRB~221009A as the template: first, that they all share the same intrinsic VHE spectral shape; second, they have the same environmental parameter and electron spectral index, governing their synchrotron self-Compton (SSC) emission. Using the long GRB luminosity and redshift distribution function obtained from the Fermi-GBM GRB samples, and accounting for the cosmological effects and extra-galactic background light (EBL) absorption, we derive the expected VHE flux at Earth. The sensitivity analysis for LHAASO, the upcoming LACT, and SWGO to evaluate their detection potential across specific redshift and luminosity ranges has been performed. The corresponding 5$σ$ detection rates of 221009A-like GRBs for the two emission models are: LHAASO, 0.04-0.05 yr$^{-1}$; LACT, 0.03-0.06 yr$^{-1}$; SWGO, 0.2-0.4 yr$^{-1}$. These rates can vary by up to $\approx 24\%$ due to different EBL models.

Figures (5)

• Figure 1: The contours of $\log(TS)$ for 221009A-like GRBs across a range of luminosities and redshifts, calculated for LHAASO‑WCDA, LHAASO‑KM2A, LACT, and SWGO. All panels assume the most probable prompt duration $T_{90} = 28$ s. and a 68% containment of the point‑spread function. Solid and dashed black lines mark the TS = 25 and TS = 9 boundaries for the power‑law model, corresponding to $5\sigma$ and $3\sigma$ detection regions, respectively. Similarly, solid and dashed green lines indicate the corresponding boundaries for the SSC model. See the main text for further details.
• Figure 2: The detectability of VHE emission from 221009A-like GRBs at the $5\sigma$ level is shown in the redshift–luminosity plane for LACT, WCDA, KM2A, and SWGO, assuming the most probable prompt duration $T_{90} = 28$ s. Here, the luminosity represents the peak luminosity in the 1-$10^4$ keV band for each simulated GRB. The red solid, orange dashed, blue dash‑dotted, and black dotted curves correspond to LHAASO‑WCDA, LHAASO‑KM2A, LACT (with 32 IACTs), and SWGO, respectively. For each experiment, the detectability is evaluated based on the expected number of events within the 68% containment radius of the point-spread function.
• Figure 3: SED of GRB 221009A from the SSC model fit to the LHAASO (KM2A and WCDA) and AGILE data in two time intervals. The MCMC 1$\sigma$-confidence band is shown over the model for the best-fit parameter reported in the main text.
• Figure 4: Corner plot showing the posterior distributions of the SSC model parameters obtained from an MCMC fit to the LHAASO and AGILE data of GRB 221009A in two time intervals as in Fig. \ref{['fits2']}.
• Figure 5: Effective area after p/$\gamma$ discrimination for LHAASO, LACT (32 IACTs) and SWGO . The red solid curve shows the gamma-ray effective area of WCDA; the orange dotted curve corresponds to KM2A the_lhaaso_collaboration_very_2023. The blue dash-dotted curve represents the collection area of LACT with 32 IACTs zhang_layout_2025, and the black dotted line indicates the effective area of SWGO collaboration_science_2025-1.