2024MNRAS.527.8140A

We present analytical and numerical models of the bright long GRB 210822A at z = 1.736. The intrinsic extreme brightness exhibited in the optical, which is very similar to other bright GRBs (e.g. GRBs 080319B, 130427A, 160625A 190114C, and 221009A), makes GRB 210822A an ideal case for studying the evolution of this particular kind of GRB. We use optical data from the RATIR instrument starting at T + 315.9 s, with publicly available optical data from other ground-based observatories, as well as Swift/UVOT, and X-ray data from the Swift/XRT instrument. The temporal profiles and spectral properties during the late stages align consistently with the conventional forward shock model, complemented by a reverse shock element that dominates optical emissions during the initial phases (T < 300 s). Furthermore, we observe a break at T = 80 000 s that we interpreted as evidence of a jet break, which constrains the opening angle to be about θ_j = (3-5) degrees. Finally, we apply a machine-learning technique to model the multiwavelength light curve of GRB 210822A using the AFTERGLOWPY LIBRARY. We estimate the angle of sight θ_obs = (6.4 ± 0.1) × 10^–1 degrees, the energy E₀ = (7.9 ± 1.6) × 10⁵³ erg, the electron index p = 2.54 ± 0.10, the thermal energy fraction in electrons ϵ_e = (4.63 ± 0.91) × 10^–5 and in the magnetic field ϵ_B = (8.66 ± 1.01) × 10^–6, the efficiency χ = 0.89 ± 0.01, and the density of the surrounding medium n₀ = 0.85 ± 0.01 cm^–3.