The gravitational wave detection of a binary neutron star merger: expectations, surprises, and prospects

On August 17, 2017, Advanced LIGO & Virgo detected gravitational waves from a binary neutron star merger. A short-duration gamma-ray burst was detected in temporal coincidence by the INTEGRAL and Fermi satellites. A few hours later, an optical/NIR transient was found which turned out to be compatible with the predictions of a kilonova, powered by the radioactive decay of heavy r-process nuclei produced in the merger. I will give an overview of the observational results of this event which will go down in history as the start of multi-messenger gravitational wave astronomy. I will contrast the original expectations with the actual findings, and will spend most of the time on the mismatches, i.e. the new questions which emerged.