First came the gravitational waves. Then, a burst of gamma radiation. As night fell in Chile, a small telescope had pinpointed the signals in the sky: the first ever neutron star smash-up found with gravitational waves.
Hours after the first signal in the Laser Interferometer Gravitational-Wave Observatory (LIGO) detector in Hanford, Washington, on 17 August, about 70 telescopes and observatories across the planet and in space turned in concert to face the same spot in the constellation Hydra.
“I don’t think it’s out of the question that this is the most observed astronomical event ever. It’s a thrilling notion, and a little overwhelming,” says LIGO spokesperson David Shoemaker. “We’ve got somewhere between a quarter and a third of all the world’s astronomers working with us.”
It marks the first gravitational waves from something other than a black hole binary, the first proof that neutron star mergers can cause gamma ray bursts, the first sighting of heavy elements being formed and the first measurement of the universe’s expansion using gravitational waves.
Since it first started listening for disturbances in space-time, LIGO has heard five signals from gravitational waves created as pairs of black holes merged and sent ripples throughout the universe, but none from any other cosmic characters.
“We’d always thought that we would see binary neutron stars first and we finally did [see them], and it was every bit as spectacular as we hoped,” says Nelson Christensen at Carleton College in Minnesota.
The new Virgo detector in Italy didn’t directly see the gravitational waves because they were in its blind spot. But that helped pinpoint its location to an area small enough to search with optical telescopes. “It was like a dart game,” says Christensen. “We just kept narrowing it down and then we finally got a bullseye with the optical detection.”
The smash-up was seen in wavelengths across the electromagnetic spectrum, from radio to gamma rays. It is the first time an event has been observed in both gravitational waves and light.
The gravitational waves give us an idea of the inner workings of the collision, while the images in different wavelengths of light, including pictures from the Hubble Space Telescope, show us the resulting cloud of hot plasma and gas, and let us pin down the location.
Because neutron stars are much smaller than black holes – in this case, each was less than twice the mass of the sun – they need to be far closer for us to see the ripples they create in space-time as they merge. This pair were only about 130 million light years away, more than 10 times closer than the next-nearest gravitational wave source we have seen.
The waves produced in this cosmic collision also revealed the speed at which the universe is expanding. Normally, this is calculated by measuring how fast supernovae or stars with well-known luminosity – called standard candles – are moving away from us. Instead, LIGO used gravitational waves, confirming previous calculations of this speed.
“Usually we measure the expansion of the universe with standard candles and now we have a standard siren,” says LIGO deputy spokesperson Laura Cadonati.
Based on gravitational waves alone, we can’t tell for sure what type of objects are sending waves our way – LIGO can only tell their size and velocity. These two objects were between one and two times the mass of the sun, so they could have been either neutron stars or small black holes.
But observations in all of the wavelengths prove that they are indeed neutron stars.
“The collision spews out material from the surfaces of the objects, which cannot happen if one or both of them do not have a surface – and black holes don’t have a surface, they have an event horizon,” says Edo Berger at Harvard University.
“It’s this 11-billion-year-long dance of these objects orbiting around each other, and then at the moment of collision all hell breaks loose,” Berger says.
In the explosion, called a kilonova, astronomers saw signs for the first time of heavy elements being formed as neutrons spewed out of the merging stars and hit lighter atoms nearby.
“People have long suspected that heavy elements were made in neutron star mergers, but this is really the first time we’ve nailed that down,” says Andrew Levan at the University of Warwick, UK. This merger made something like the mass of the Earth in gold, he says, along with other heavy elements such as platinum, lead and uranium.
What is left behind after the collision remains shrouded in a cloud of heavy elements and mystery. Once the two stars smashed together, they may have formed a larger neutron star or collapsed into a black hole.
Berger says many of the astronomers involved in the discovery think the collision resulted in a black hole, and that the gamma ray burst that occurred just afterwards was powered by matter being sucked into it. However, there is no way yet to know for sure.
But we now know we have been witnessing neutron star mergers for years – for as long as we’ve been observing short gamma ray bursts – without knowing it. The explosion of gamma radiation that resulted as the latest pair of neutron stars first touched is the closest we have ever seen, and it is similar to a few others from further across the universe.
“The kilonova goes on for a long, long time – days to weeks, or months,” says Shane Larson at Northwestern University in Illinois. “We’re going to be able to map it out in extreme detail because observatories around the world are still monitoring it.”
And as LIGO increases in sensitivity over the next few years, it could see up to one neutron star merger every week in addition to spotting even more exotic signals from objects such as supernovae or cosmic strings, Cadonati says.
“There will be even more surprises when we see more than this one event, but the first one always has a special place,” says Berger. “This is one of those instances where a single event includes so many firsts that it’s hard to keep track.”
Perhaps most important, it is the first time that we have both seen the light and listened to the vibrations in space-time caused by one cosmic collision. We have entered an era of observing the universe with all our senses at once.