Theoretical Modeling of the Exceptional GRB 221009A Afterglow

Abstract The extraordinary gamma-ray burst GRB 221009A provides a great opportunity to investigate the enigmatic origin and evolution of gamma-ray bursts (GRBs). However, the complexity of the observations associated with this GRB provides significant challenges to developing a theoretical modeling in a coherent framework. In this paper, we present a theoretical interpretation of the GRB 221009A afterglow within the relativistic fireball scenario, aiming to describe the broadband data set with a consistent model evolution. We find that the adiabatic fireball evolution in the slow-cooling regime provides a viable scenario in good agreement with observations. Crucial to our analysis is the set of simultaneous GeV and TeV gamma-ray data obtained by AGILE and LHAASO during the early afterglow phases. Having successfully modeled as inverse Compton emission the high-energy spectral and lightcurve properties of the afterglow up to 10 4 s, we extend our model to later times when also optical and X-ray data are available. This approach results in a coherent physical framework that successfully describes all observed properties of the afterglow up to very late times, approximately 10 6 s. Our model requires time-variable microphysical parameters, with a moderately increasing efficiency ε e of a few percent for transferring the shock energy to radiating particles and a decreasing efficiency for magnetic field generation ε B in the range 10 −5 –10 −7 . Fitting the detailed multifrequency spectral data across the afterglow provides a unique test of our model.