// block 480929 · science · 17 August 2017

The first neutron star merger observed

The moment this block marks: the signal GW170817 reaches the LIGO and Virgo detectors, 17 August 2017, 12:41:04 UTC. The first block mined after it is block 480,929, 17 August 2017, 12:50:15 UTC.

On 17 August 2017, the two LIGO detectors in the United States and the Virgo detector near Pisa, Italy, recorded a signal unlike any they had seen. The earlier detections, from merging black holes, had lasted a fraction of a second. This one lasted about 100 seconds: the long spiral of two much lighter objects, neutron stars, the collapsed cores of massive stars, each between 1.17 and 1.60 times the mass of the Sun, some 130 million light-years away.

Just 1.7 seconds after the gravitational waves, the Fermi space telescope recorded a short burst of gamma rays from the same part of the sky. Within twelve hours, astronomers in Chile, who had spent most of that time waiting for nightfall, found a new, fading point of light at the edge of the galaxy NGC 4993. Some 70 observatories went on to follow it, from gamma rays to radio waves.

It was the first time a single cosmic event had been seen both in gravitational waves and in light, and it gave the first direct evidence that neutron star mergers produce short gamma-ray bursts. The results were announced on 16 October 2017.

The first block mined after the signal arrived, block 480,929, came nine minutes later, at 12:50 UTC, long before the glow in NGC 4993 had been found. Its hash, like every hash, can be checked on any node.

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Sources

• B. P. Abbott et al. (LIGO and Virgo), GW170817: Observation of Gravitational Waves from a Binary Neutron Star Inspiral, Physical Review Letters 119, 161101: “On August 17, 2017 at 12:41:04 UTC”
• LIGO Caltech, LIGO and Virgo make first detection of gravitational waves produced by colliding neutron stars, 16 October 2017