Determining the viewing angle from TeV light curve of GRB 221009A

Gamma-ray bursts (GRBs) are among the most powerful explosive events in the Universe. LHAASO recently observed the most luminous one: GRB 221009A, and unveiled its TeV light curve. The light curve exhibits a distinct jet break at around 670 s, enabling the derivation of the viewing angle based on the smoothness of the jet break. We constructed two models with or without considering the high-latitude radiation, where the viewing angle was treated as a free parameter, to fit the TeV light curve. The viewing angles obtained were 9.4 × 10⁻⁴ and 5.9 × 10⁻³ rad, respectively. These values closely resemble an on-axis scenario, given the opening angle is 1.4 × 10⁻² rad.