Superluminal motion, successful jet and link with short GRBs from the radio afterglow

Radio afterglows of neutron star mergers inform us about the geometry and energy of the different ejecta components, and the physics of the shock occurring between the ejecta and the circum-burst environment. This information is complementary to the ejecta mass and composition information provided by the early-time optical emission arising from the r-process nucleosynthesis of the neutron-rich dynamical ejecta (i.e. the kilonova/macronova signature). The radio afterglow of GW170817 was first detected 16 days after the merger. It evolved slowly with time (t^0.8), peaked at about 150 days post-merger, and declined steeply (t^-2) thereafter, unlike any short-hard gamma-ray burst (SGRB) afterglow. In this talk I will give an overview of the radio discovery of GW170817, the properties of its radio afterglow, and present results from VLBI observations that finally break the degeneracy between the choked-jet and successful-jet models for GW170817, thus unambiguously linking binary neutron star mergers and SGRBs. GW170817 represents only an initial exploration of a rich scientific landscape populated by the stellar evolution, explosion and eventual merger of massive binary systems. The study of radio afterglows of neutron star mergers detected by LIGO/Virgo in their upcoming observing runs will be able to address many unsolved questions such as: How much energy do mergers release? What fraction of mergers produce relativistic jets? What effect does viewing angle have on the afterglow?

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