GW170817 was a GW Detection by LIGO and Virgo for which a corresponding Gamma-Ray Burst was detected (a short burst, i.e.,SGRB 1.7 seconds later, GRB 170817A), and for which the source was found, through checking for Transients in the region of the Celestial Sphere (e.g., with the Víctor M. Blanco Telescope DECam) indicated through triangulation from the three detectors. As such, it is considered the first case of Multi-messenger Astronomy (observation via EMR as well as other media). Virgo's detection was weak (i.e., a low Signal-To-Noise Ratio) and the fact that detector sensitivity depends upon the direction of the source was a factor in reducing the region of the sky to search.
The source was in the Galaxy NGC 4993.
The event has been interpreted as a Neutron Star Merger due to the pattern of Gravitational Waves. It is observationally somewhat like a Supernova, a transient, but showing less EMR-power with a different subsequent evolution, falling under the class of Kilonova (a SN-like transient but with less power). The event has been the subject of much study, being the first such observation in which the underlying event is so certain. Among the revelations:
The r-process is expected as the shock of impact sends neutrons flying free, and is detected through the EMR produced by hot Elements, heated for a while by radioactivity (e.g., Beta Decay), all consistent with the products of r-process Nucleosynthesis. Among the effects is high Opacity soon after the merger, due to the elements heavier than Iron.
Early EMR observation appeared to be made unclear by the effects of Relativistic Speed of ejected material.
Given the scenario most likely to form a Binary Neutron Star, the decay of its orbit would take a very long time, and estimates are that this system's orbit decayed over 11 gigayears leading up to the merger.