GW170817 was a gravitational wave (GW) signal observed by the LIGO and Virgo detectors on 17 August 2017, originating from the shell elliptical galaxy NGC 4993, about 144 million light years away.

[6][10] The gravitational wave signal, designated GW170817, had an audible duration of approximately 100 seconds, and showed 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.

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.

[8] It was captured by numerous telescopes, from radio to X-ray wavelengths, over the following days and weeks, and was found to be a fast-moving, rapidly-cooling cloud of neutron-rich material, as expected of debris ejected from a neutron-star merger.

The similarities between the two events in terms of gamma ray, optical, and x-ray emissions, as well as to the nature of the associated host galaxies, 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 kilonova transients, making kilonovae more diverse and common in the universe than previously understood.

[5][11][8] Some information was leaked before the official announcement, beginning on 18 August 2017 when astronomer J. Craig Wheeler of the University of Texas at Austin tweeted "New LIGO.

Other people followed up on the rumor, and reported that the public logs of several major telescopes listed priority interruptions in order to observe NGC 4993, a galaxy 40 Mpc (130 Mly) away in the Hydra constellation.

It covered approximately 3,000 cycles, increasing in amplitude and frequency to a few hundred hertz in the typical inspiral chirp pattern, ending with the collision received at 12:41:04.4 UTC.

[22][2] In particular, the absence of a clear detection by the Virgo interferometer implied that the source was localized within one of its blind spots, a constraint which reduced the search area considerably.

This GRB was relatively faint given the proximity of the host galaxy NGC 4993, possibly due to its jets not being pointed directly toward Earth, but rather at an angle of about 30 degrees off axis.

[23] In total six teams (One-Meter, Two Hemispheres (1M2H),[27] DLT40, VISTA, Master, DECam, and Las Cumbres Observatory (Chile)) imaged the same new source independently in a 90-minute interval.

[2][1] A possible explanation for the non-detection of neutrinos is because the event was observed at a large off-axis angle and thus the outflow jet was not directed towards Earth.

[2] If low spins are assumed, consistent with those observed in binary neutron stars expected to merge within (twice[a]) the Hubble time, the total mass is 2.74+0.04−0.01 M☉.

[45] The short gamma-ray burst was followed over the next several months by its slower-evolving kilonova counterpart, a spherically expanding optical afterglow powered by the radioactive decay of heavy r-process nuclei produced and ejected at the initial cataclysmic instant.

[46][47] GW170817 therefore confirmed neutron star mergers to be viable sites for the r-process, where the neucleosynthesis of around half the isotopes in elements heavier than iron can occur.

[49] However, a more detailed analysis of the GW170817 signal tail later found evidence of further features consistent with the seconds-long spindown of an intermediate or remnant hypermassive magnetar,[50] the energy of which was below the estimated sensitivity of the LIGO search algorithms at the time.

[2] The limits of possible violations of Lorentz invariance (values of 'gravity sector coefficients') are reduced by the new observations by up to ten orders of magnitude.

[44] The event also excluded some alternatives to general relativity,[57] including variants of scalar–tensor theory,[58][59][60][61][62][63][64][65] Hořava–Lifshitz gravity,[61][66][62] Dark Matter Emulators,[67] and bimetric gravity,[68] Furthermore, an analysis published in July 2018 used GW170817 to show that gravitational waves propagate fully through the 3+1 curved spacetime described by general relativity, ruling out hypotheses involving "leakage" into higher, non-compact spatial dimensions.