Physical properties of the brightest gamma-ray burst based on observations by the Fermi Gamma-ray Burst Monitor

GRB 221009A was the brightest gamma-ray burst (GRB) detected since the start of GRB observations. Even though it saturated Fermi-GBM, it presents a rich and unique data set to test theoretical models of gamma-ray production. We present spectral analysis of the pulses not affected by instrumental saturation and confront the synchrotron emission model with the observations. Interestingly, the afterglow is also detected by GBM. We determine the transition from prompt emission to afterglow, and derive the Lorentz factor of the outflow using multiple methods.