Puzzling two-stage size evolution of an ultraluminous gamma-ray burst jet

Abstract Gamma-ray bursts (GRBs) are the most luminous stellar explosions in the Universe, in which relativistic jets are involved. GRB 221009A stands out as the brightest GRB ever observed, providing a unique opportunity to probe the properties of an ultrarelativistic jet. Here, we report our very long baseline interferometry (VLBI) observation on the extremely bright GRB 221009A from 5 to 26 days after the burst. An intriguing two-stage evolution of the source size of this GRB is revealed by combining this data with later VLBI data. The observed size initially increases slowly as ~ tobs0.15 during the first month, but expands with a much faster rate as ~ tobs2.21 at later times. This unprecedented two-stage size evolution cannot be explained by the existing theoretical models invoking a non-spreading top-hat jet or a conventional structured jet and thus challenges our current understanding of this GRB jet. The transition may suggest the beginning of rapid sideways expansion of a narrow jet as the relativistic jet decelerates enough or the emergence of a new emission component. Our findings unveil the complex jet dynamics of GRB 221009A, highlighting the necessity of direct measurements of the size and geometry of the emission region in studying low-redshift GRBs in the multi-wavelength era.