GW170817 (2017): The Neutron Star Merger Seen in Waves and Light

GW170817 was the first merger of two neutron stars ever detected, observed by LIGO and Virgo on August 17, 2017. Gravitational waves pinpointed it; 1.7 seconds later a gamma-ray burst arrived, and within 11 hours telescopes found its glowing debris, a kilonova, in the galaxy NGC 4993. It launched multi-messenger astronomy.

The question before the paper

The Hulse-Taylor binary pulsar had shown that pairs of neutron stars, the ultra-dense remnants of exploded stars, slowly spiral together by emitting gravitational waves. Their final collision should be detectable by LIGO. Theorists had long suspected such collisions might also produce short gamma-ray bursts and forge heavy elements. After LIGO’s first detection of merging black holes in 2015, the next question was whether a neutron star merger could be caught, and seen with telescopes too.

What the paper found

• The first binary neutron star inspiral. At 12:41:04 UTC on August 17, 2017, Advanced LIGO and Advanced Virgo recorded the signal of two neutron stars spiraling together, with a combined signal-to-noise ratio of 32.4.
• Neutron star masses. The components weighed 1.17 to 1.60 solar masses each (assuming low spins), about 2.74 solar masses in total.
• Close and well located. The source lay about 40 megaparsecs (roughly 130 million light-years) away, localized to 28 square degrees of sky, the closest and best-localized gravitational-wave source at the time.

How they did it

The two LIGO detectors and Virgo in Italy observed together. Neutron stars are lighter than the black holes of GW150914, so the signal lasted much longer in the detectors’ frequency band, letting the pair’s masses be measured well. Combining the signal’s arrival times and strengths at three detectors narrowed its position on the sky to a patch small enough for telescopes to search.

Why it mattered

The same event was seen across the spectrum. Fermi’s Gamma-ray Burst Monitor detected GRB 170817A 1.7 seconds after the merger, linking neutron star mergers to short gamma-ray bursts. A joint analysis used that delay to show that gravitational waves and light travel at the same speed to within a few parts in 10¹⁵. Less than 11 hours later, telescopes found the optical counterpart, a kilonova named AT 2017gfo, in the galaxy NGC 4993. Its spectra showed freshly made heavy elements from the r-process, the rapid neutron-capture process. Neutron star collisions forge a large share of the universe’s heavy elements, such as gold.

Combining the gravitational-wave distance with the host galaxy’s redshift gave a “standard siren” measurement of the Hubble constant of about 70 km/s per megaparsec, a new route into the Hubble tension.

What we know now

The LIGO-Virgo-KAGRA network has since confirmed 390 events in its GWTC-5.0 catalog (May 2026), mostly black hole mergers; see GWTC-4. Better sky localization for future mergers will come as LIGO-India joins the network. See our gravitational waves guide.

Who wrote it

The paper is signed by the LIGO Scientific Collaboration and the Virgo Collaboration, with individual authors starting with B. P. Abbott and R. Abbott. Companion papers described the gamma-ray burst, the kilonova and the Hubble constant. This one has about 2,300 citations in Semantic Scholar as of October 2026.

Read the original

LIGO Scientific Collaboration and Virgo Collaboration, Abbott, B. P., Abbott, R., et al. (2017). “GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral.” Physical Review Letters 119, 161101. doi:10.1103/PhysRevLett.119.161101. Free on arXiv: 1710.05832. The standard siren paper is arXiv:1710.05835.

Questions people ask

Do gravitational waves travel at the speed of light?

Yes, as far as anyone can measure. The gamma rays from GW170817 arrived about 1.7 seconds after the gravitational waves, after traveling for some 130 million years. A companion LIGO, Virgo and Fermi analysis constrained any speed difference to less than a few parts in 10¹⁵ of the speed of light.

What is a kilonova?

A kilonova is the glow of hot debris thrown out when two neutron stars merge, rich in freshly made heavy elements. GW170817’s kilonova, AT 2017gfo, was found less than 11 hours after the merger, and its spectra showed elements made by rapid neutron capture, the r-process.