GW170817

<a href="/facts/Celestial_coordinate_system/23HhqvHj">Coordinates</a>: <a href="http://www.wikisky.org/?ra=13.163355555556&de=-23.381472222222&zoom=9&show_grid=1&show_constellation_lines=1&show_constellation_boundaries=1&show_const_names=1&show_galaxies=1&img_source=IMG_all">13h 09m 48.08s, −23° 22′ 53.3″</a>

GW170817 was a <a href="/facts/Gravitational_wave/4XWN3NtG">gravitational wave</a> (GW) observed by the <a href="/facts/LIGO/jX6jyJzM">LIGO</a> and <a href="/facts/Virgo_interferometer/wfaxuUqc">Virgo</a> detectors on 17 August 2017, originating from the shell elliptical galaxy <a href="/facts/NGC_4993/1k5paXtv">NGC 4993</a>, about 144 million light years away. The wave was produced by the last moments of the <a href="/facts/Inspiral/c9rj3aK6">inspiral</a> of a binary pair of <a href="/facts/Neutron_star/4enfoxBA">neutron stars</a>, ending with their <a href="/facts/Neutron_star_merger/AMLBIl3G">merger</a>. As of April 2025<a href="https://en.wikipedia.org/w/index.php?title=GW170817&action=edit">[update]</a>, it is the only GW detection to be definitively correlated with any <a href="/facts/Electromagnetic/oAkmALHT">electromagnetic</a> observation. 
Unlike the five prior GW detections—which were of merging <a href="/facts/Black_hole/1r2lmkcF">black holes</a> and thus not expected to have detectable electromagnetic signals—the aftermath of this merger was seen across the <a href="/facts/Electromagnetic_spectrum/24LARvFY">electromagnetic spectrum</a> by 70 observatories on 7 continents and in space, marking a significant breakthrough for <a href="/facts/Multi-messenger_astronomy/7pvCx4PL">multi-messenger astronomy</a>. The discovery and subsequent observations of GW170817 were given the <a href="/facts/Breakthrough_of_the_Year/c4uKyAcJ">Breakthrough of the Year</a> award for 2017 by the journal <a href="/facts/Science_(journal)/uTyzjkwD">Science</a>.
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 <a href="/facts/Direction_finding/pAtU7f0I">direction to the source</a>. Independently, a <a href="/facts/Short_gamma-ray_burst/oRRU6Fhn">short gamma-ray burst</a> (sGRB) of around 2 seconds, designated GRB 170817A, was detected by the <a href="/facts/Fermi_Gamma-ray_Space_Telescope/rwKktSFn">Fermi</a> and <a href="/facts/INTEGRAL/4PeM0mJc">INTEGRAL</a> 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 gravitational wave detection for the expected emission at optical wavelengths. During this search, 11 hours after the signal, an <a href="/facts/Astronomical_transient/aqVQg1Nm">astronomical transient</a> SSS17a, later designated <a href="/facts/Kilonova/okpV1akI">kilonova</a> AT 2017gfo, was observed in the galaxy <a href="/facts/NGC_4993/1k5paXtv">NGC 4993</a>. 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.
In October 2018, astronomers reported that, in retrospect, an sGRB event detected in 2015 (<a href="/facts/GRB_150101B/DR9mVu3m">GRB 150101B</a>) may represent an earlier case of the same astrophysics reported for GW170817. The similarities between the two events in terms of <a href="/facts/Gamma_ray/5Drf913J">gamma ray</a>, <a href="/facts/Optical/p0vq4S1T">optical</a>, and <a href="/facts/X-ray/wseP79cN">x-ray</a> emissions, as well as to the nature of the associated host <a href="/facts/Galaxy/zCyUvEf7">galaxies</a>, were considered "striking", suggesting that the earlier event may also be the result of a neutron star merger, and that together these may signify a hitherto-unknown class of <a href="/facts/Kilonova/okpV1akI">kilonova</a> transients, making kilonovae more diverse and common in the universe than previously understood. Later research further construed GRB 160821B—another sGRB predating GW170817—also to belong to this class, again based on afterglow resemblance to the AT 2017gfo signature.

GW170817 open-in-new

GW170817