GW170817

GW170817 was a gravitational wave (GW) observed by the LIGO and Virgo detectors on 17 August 2017, originating within the shell elliptical galaxy NGC 4993, about 144 million light years away. The wave was produced by the last moments of the inspiral of a binary pair of neutron stars, ending with their merger. As of June 2025, it is the only GW detection to be definitively correlated with any electromagnetic observation.

Unlike the five prior GW detections—which were of merging black holes and thus not expected to have detectable electromagnetic signals—the aftermath of this merger was seen across the electromagnetic spectrum by 70 observatories on 7 continents and in space, marking a significant breakthrough for multi-messenger astronomy. The discovery and subsequent observations of GW170817 were given the Breakthrough of the Year award for 2017 by the journal Science.

GW170817 had an audible duration of approximately 100 seconds and exhibited the characteristic intensity and frequency expected of the inspiral of two neutron stars. Analysis of the slight variation in arrival time of the GW at the three detector locations (two LIGO and one Virgo) yielded an approximate angular direction to the source. Independently, a short gamma-ray burst (sGRB) of around 2 seconds, designated GRB 170817A, was detected by the Fermi and INTEGRAL spacecraft beginning 1.7 seconds after the GW emitted by the merger. These detectors have very limited directional sensitivity, but indicated a large region of the sky which overlapped the gravitational wave direction. The co-occurrence confirmed a long-standing hypothesis that neutron star mergers describe an important class of sGRB progenitor event.

An intense observing campaign was prioritized, to scan the region indicated by the sGRB/GW detection for the expected emission at optical wavelengths. During this search, 11 hours after the signal, an astronomical transient SSS17a, later designated kilonova AT 2017gfo, was observed in the galaxy NGC 4993. It was captured by numerous telescopes in other electromagnetic bands, from radio to X-ray wavelengths, over the following days and weeks. It was found to be a fast-moving, rapidly-cooling cloud of neutron-rich material, as expected of debris ejected from a neutron-star merger.