Constraints on Axion-like Particles from the Observation of GRB 221009A by LHAASO

Abstract: The LHAASO collaboration recently reported the measurement of the gamma-ray spectra of GRB 221009A, which is the brightest burst ever, covering an energy range from 0.3 $\mathrm{TeV}$ to about 10 $\mathrm{TeV}$. Based on the observation, we investigate the ALP-photon oscillation effect in the host galaxy of GRB 221009A and the Milky Way. The ${\rm CL_s}$ method is applied to set constraints on the ALP parameters in this study. Given the uncertain magnetic field configuration in the host galaxy, we use three different models: a homogeneous magnetic field model, a magnetic field model identical to that of the Milky Way, and a model constructed from the HST observations of the host galaxy. We find that the constraints derived using these three host galaxy magnetic field models are comparable. Our results are complementary in the small ALP mass regions compared with other experiments.