The role of VLBI observations as multi-messenger probes of compact binary mergers

The Advanced LIGO ground-based interferometric gravitational wave (GW) detectors revolutionized our view on the Universe by demonstrating for the first time in 2015 the very existence of GW with properties in agreement with the predictions of General Relativity and the feasibility of their detection. In 2017, in a network with the Advanced Virgo detector, they detected and localized the first ever GW + photon source, the binary neutron star (BNS) merger GW170817, opening a new era in multi-messenger science. VLBI observations proved fundamental and complementary in the characterization and understanding of that source: they settled the debate on whether a narrow, relativistic jet was produced after the BNS merger, with important implications on the equation of state of neutron star matter; they allowed for breaking the inherent distance-inclination degeneracy in the GW signal, allowing for the first several-percent-precision multi-messenger determination of the Hubble constant. The O4 observing run of the ground-based GW detector network is about to start, with an increased projected detection rate of BNS mergers and also, potentially, black hole - neutron star mergers. VLBI observations will likely play again an important role in unveiling the secrets of these systems.