Multibands fitting of Gamma-ray burst’s afterglow’s light curves using the synchrotron external forward shock model

The Swift Gamma-Ray Bursts (GRBs) satellite has observed GRB afterglows with unprecedented resolution over the last two decades, using instruments like the Burst Alert Telescope (BAT), the X-Ray Telescope (XRT), and the Ultra Violet-Optical Telescope (UVOT). This has motivated intensive modeling efforts to interpret these observations, revealing complex light curves with multiple emission mechanisms. Besides two-dimensional hydrodynamical simulations, recent work uses the asymptotic spectral approach to fit observed GRB afterglow light curves and constrain physical parameters. Analytical models, which treat jet dynamics in detail, are rarely used, and their ability to reproduce recent observations is undervalued. This study uses the analytical external shock model to test its ability to perform a joint X-ray and optical/near-Infra-Red (IR) fit to a sample of GRB afterglows light curves chosen based on low/moderate complexity of observed X-ray emissions and availability of their optical/near-IR counterparts. We developed a numerical code that simultaneously solves the fireball model with synchrotron emission for a set of microphysical parameters and performs a Monte-Carlo multi-band fitting analysis based on \(\chi ^{2}\) minimization. Results showed reasonable agreement with 70% of the data, with both homogeneous interstellar medium (ISM) and stellar wind (WIND) density models providing relatively equal quality fits, slightly deviating from the data in the latter time evolution phase. Best-fit values of microphysical parameters are consistent with previous results, although they show significant degeneracy leading to mutual correlations. The correlation study confirms the ability of the multi-band fitting to break the degeneracies and increase constraints on physical parameters.