Kilonova Emission and Heavy Element Nucleosynthesis

The binary neutron star merger observed and localized on 17 August 2017 by the LIGO and Virgo gravitational interferometers and by numerous telescopes on the ground and in orbit linked in an unambiguous way the coalescence of double neutron stars with the formation of a relativistic outflow (short gamma-ray burst GRB170817A) and of a thermal radioactive source (kilonova). The vicinity of the event (40 Mpc) made it possible to monitor the electromagnetic counterpart in detail at all wavelengths and to map its close environment in the outskirts of the lenticular galaxy NGC 4993. Radio VLBI images of GRB170817A allowed the first direct detection of superluminal motion in a GRB afterglow, pointing to a collimated ultra-relativistic jet rather than to a quasi-isotropically, mildly relativistically expanding source. The accurate spectroscopy of the kilonova at ultraviolet-to-infrared wavelengths with the X-Shooter spectrograph of the ESO Very Large Telescope showed the long-sought-after signature of rapid neutron capture process (in short: r-process) nucleosynthesis. Kilonova detection makes gravitational wave sources optimal tracers of heavy element formation sites.