Multi-messenger Observations of a Binary Neutron Star Merger

On 2017 August 17 the Advanced LIGO and Advanced Virgo interferometers observed the merger of a binary neutron star system. The INTEGRAL SPI-ACS and Fermi-GBM instruments independently observed the short Gamma Ray Burst GRB 170817A, after a delay of about 1.7 s. GW170817 was localized within a sky region of about 30 deg2 at a luminosity distance of 40 Mpc. A worldwide electromagnetic follow-up campaign detected an optical counterpart (SSS17a/AT 2017gfo) in the elliptical galaxy NGC 4993. The optical and infrared spectroscopic observations during the first days after the merger showed the signature of a kilonova. The first X-ray and radio detections of the afterglow occurred at 9 and 16 days after the merger, respectively. The multi-messenger observations suggest that GW170817 was produced by the merger of two neutron stars in the galaxy NGC 4993, followed by the short gamma ray burst GRB 170817A and by a kilonova powered by the radioactive decay of r-process nuclei synthesized in the ejecta. This review is an update of [110], summarizing the multi-messenger observations of GW170817 from the discovery to the late stages.