Exponential decay vs. power-law decay for the prompt emission of GRB 221009A

The pulse profile of Gamma-ray bursts (GRBs) is often described as fast rise and exponential decay (FRED). However, the exponential decay feature can be decisively identified from power-law decay only if the flux decays several orders of magnitude. GRB 221009A, the brightest GRB ever observed, should be able to test the FRED feature. Using data from Fermi-GBM, we examined the decay pattern of the burst across different energy ranges. Power-law and exponential models were fitted to different segments of the decay phase, and the posterior distributions of the model parameters were estimated using Markov Chain Monte Carlo (MCMC) method. The power-law decay fits the light curve better than the exponential decay in both the precursor and the main burst. This implies that they may originate from the same physical mechanism, which may improve our understanding of the mechanisms of GRB emission and the dynamics of the central engine.