GW170817
/ G-W one-seven-oh-eight-one-seven /
If GW150914 proved we could hear black holes, GW170817 proved we could hear and see the same cosmic event at once. On 17 August 2017, gravitational-wave detectors felt two neutron stars spiral together about 130 million light-years away. Just 1.7 seconds after the waves ended, a satellite caught a short gamma-ray burst from the same direction — and within hours, telescopes around the world found the glowing point of light it left behind. It was the first cosmic event observed in both gravitational waves and light.
The gravitational-wave chirp identified the colliding objects as neutron stars (much lighter than the black holes of GW150914), and the short gamma-ray burst confirmed at last that such mergers cause short bursts. Then, over the following days, the fading visible and infrared glow — a kilonova — showed the unmistakable signature of freshly forged heavy elements, including gold and platinum, being flung into space. Dozens of observatories on the ground and in orbit, watching across the spectrum, turned one collision into the most richly documented event in astrophysics.
GW170817 matters because it tied many threads together at a single stroke. It confirmed that neutron-star mergers make short gamma-ray bursts and forge much of the universe's heavy elements; by comparing the wave's distance with the host galaxy's recession, it gave an independent measure of the cosmic expansion rate; and because the gravitational waves and the gamma rays arrived within seconds after a 130-million-year journey, it showed gravity travels at the speed of light to extraordinary precision. It is the founding triumph of multi-messenger astronomy.
Within a day of GW170817, telescopes pinpointed its glow in the galaxy NGC 4993, and as the kilonova faded its spectrum revealed the fingerprints of newly made heavy elements. A single event answered, all at once, where short gamma-ray bursts come from and where much of the universe's gold was forged.
The 2017 neutron-star merger seen in gravitational waves and light — multi-messenger astronomy's birth.
GW170817 is named for its date (2017-08-17), like all such events. It did not 'create' gold in a flash you could see; rather, the merger's debris forged heavy elements whose radioactive decay then powered the kilonova glow we observed over days.