D. Kasen et al., Origin of the heavy elements in binary neutron-star mergers from a gravitational wave event, Nature 551, 80-84 (2017)

GW170817 was the first gravitational wave detection involving the merger of two neutron stars to form a black hole.  Observations revealed an optical signal that faded after a few days, along with an infrared signal that persisted for nearly two weeks. These signals are consistent with computer model predictions for a kilonova which produces significant quantities of heavy elements via the r-process (rapid neutron capture).  The shorter-lived, spectrally featureless optical emission is compatible with an initial ejecta component composed of lighter elements, while the long-lived infrared signal is from a secondary component which is powered by the radioactive decay of heavy elements which heat the plasma.  Heavy elements with 58 < Z < 90 scatter the light strongly, leading to a long-lived emission.

See also: DOE Highlight