Studying the dynamics and the structure of Gamma-Ray Bursts with the VLBI

Some massive stars end their lives with a catastrophic explosion which leaves behind a spinning, stellar mass black hole or a highly magnetised neutron star. Regardless of the nature of the remnant, this central engine launches a jet of ionised matter which eventually interacts with the circum-burst medium through external shocks, producing the so-called Gamma-Ray Burst (GRB) afterglow, which can be detected from the very-high energy throughout the whole electromagnetic spectrum. Radio observations, and in particular VLBI, are fundamental to measure the apparent superluminal expansion (on-axis GRB) and proper motion (off-axis GRB) of the GRB outflow, to constrain its structure and to characterise the circum-burst medium. In this talk I will present two representative cases for radio studies of GRBs: GRB 201015A and GRB 221009A. For the former, interferometric observations were crucial to characterise the density profile of the circum-burst medium; on the other hand, VLBI observations of the brightest GRB of all time, GRB 221009A, allow us to constrain the expansion and the proper motion of the centroid. Finally, I will discuss how combining the SKA sensitivity and surveying capabilities with the VLBI resolution will affect GRB science