Prompt-to-afterglow transition of optical emission in a long gamma-ray burst consistent with a fireball

Long gamma-ray bursts, which indicate the end-life collapse of very massive stars, are produced by extremely relativistic jets colliding with circumstellar medium. A huge amount of energy is released both in the first few seconds, namely the internal dissipation phase, which powers prompt emissions, and in the subsequent self-similar jet-deceleration phase, which produces afterglows observed in the broadband electromagnetic spectrum. However, prompt optical emissions of gamma-ray bursts have rarely been detected, seriously limiting our understanding of the transition between the two phases. Here we report detection of prompt optical emissions from a gamma-ray burst (that is, GRB 201223A) using a dedicated telescope array with a high temporal resolution and a wide time coverage. The early phase coincident with prompt gamma-ray emissions shows a luminosity in great excess with respect to the extrapolation of gamma-rays, while the later luminosity bump is consistent with onset of the afterglow. The clearly detected transition allows us to differentiate physical processes contributing to early optical emissions and to diagnose the composition of the jet.