Multi-messenger observations of cosmic collisions and explosions: progenitors, relativistic ejecta, and remnants

The births and mergers of neutron stars and black holes, the most exotic objects in the universe, can launch the fastest cosmic jets (gamma-ray bursts; GRBs) and shake the very fabric of space-time with gravitational waves. GW170817, the merger of two neutron stars witnessed through both its gravitational wave siren and its glow at all wavelengths of light, marked the beginning of a golden age in multi-messenger astrophysics. While the GW siren of GW170817 has directly confirmed that at least some short GRBs originate from NS-NS mergers, the smoking gun linking long GRBs to core collapses i.e., the association with stripped-envelope core-collapse supernovae, has been challenged by several electromagnetic (EM) observations. In this context, I will discuss how observations at radio wavelengths can probe the ejecta and environments of compact binary mergers and extreme core collapses and help unveil their progenitors and remnants. I will then highlight opportunities and challenges ahead, as new observational facilities will transform a trickle of multi-messenger discoveries into a flood.