2025ApJ...986..170Z

The Insight Hard X-ray Modulation Telescope (Insight-HXMT) detected GRB 221009A, the brightest gamma-ray burst (GRB) observed to date, with all its three telescopes, i.e., the High Energy (HE) telescope (20-250 keV), Medium Energy (ME) telescope (5-30 keV), and Low Energy (LE) telescope (1-10 keV). Here we present the detailed observation results of all three telescopes of Insight-HXMT on the prompt emission of GRB 221009A. After dead-time and data saturation correction, we recovered the light curves of the HE, ME, and LE telescopes and find that they generally track the low-gain light curves of the third member of the Gravitational Wave High-energy Electromagnetic Counterpart All-sky Monitor (GECAM-C) that are free of data saturation issues. Particularly, the ME light curve matches the GECAM-C light curve in low gain mode above 400 keV, while the LE light curve is more consistent with the GECAM-C above 1.5 MeV. Based on simulation, we find that the signals recorded by ME and LE are actually caused by the secondary particles produced by the interaction between GRB gamma-ray photons and the material of the satellite. Interestingly, the consistency between the ME and LE light curves and GECAM-C demonstrates that ME and LE data could be used to characterize the GRB properties. Especially, the high-time-resolution light curve of ME allowed us, for the first time, to calculate the minimum variability timescale (0.10 s) of the main burst episode of GRB 221009A.