Some 130m years ago, in a galaxy far away, the smouldering cores of two collapsed stars smashed into each other. The resulting explosion sent a burst of gamma rays streaming through space and rippled the fabric of the universe. On 17 August, those signals reached Earth – and sparked an astronomy revolution.
The collision created a “kilonova”, an astronomical marvel that scientists have never seen before. It was the first cosmic event to be witnessed via both traditional telescopes, which can observe electromagnetic radiation like gamma rays, and gravitational wave detectors, which sense the wrinkles in space-time produced by distant cataclysms. The detection, which involved thousands of researchers working at more than 70 laboratories and telescopes on every continent, heralds a new era in space research known as “multimessenger astrophysics”.
“It’s transformational,” said Julie McEnery, an astrophysicist at Nasa’s Goddard Space Flight Center in Greenbelt, Maryland, who was involved in the effort. “The era of gravitational wave astrophysics had dawned, but now it’s come of age … We’re able to combine dramatically different ways of viewing the universe, and I think our level of understanding is going to leap forward as a result.”
The existence of gravitational waves was first theorised by Albert Einstein a century ago. But scientists had never sensed the waves until 2015, when a ripple produced by the merger of two distant black holes was picked up by two facilities of the Laser Interferometer Gravitational-Wave Observatory (Ligo) in Louisiana and Washington state. Since then, the collaboration has identified three more black hole collisions and has brought on a third gravitational wave detector near Pisa, Italy, to better pinpoint the sources of these distortions in space-time. This month members of the Ligo team were awarded the Nobel prize in physics.
Yet because black holes emit no light or heat, past gravitational wave detections could not be paired with observations by conventional telescopes, which collect signals from what’s known as the electromagnetic spectrum. The scientists at Ligo and its European counterpart, Virgo, hoped to detect gravitational waves from a visible event, such as a binary star merger or a kilonova.
Kilonovas are swift, brilliant explosions that occur during the merger of neutron stars, which are ultradense remnants of collapsed stars that are composed almost entirely of neutrons.
Collisions between neutron stars are thought to be 1,000 times brighter than a typical nova, and they are the universe’s primary source of such elements as silver, platinum and gold. But much like gravitational waves, kilonovas have long been strictly theoretical. Until this summer.
At 8.41am Eastern time on 17 August, a gravitational wave hit the Virgo detector in Italy and, 22 milliseconds later, set off the Ligo detector in Livingston, Louisiana. Three milliseconds after that, the distortion reached Hanford, Washington.
Ligo detects black hole mergers as quick chirps that last a fraction of a second. This signal lasted for 100 seconds, and it vibrated at higher frequencies. From the smaller amplitude of the signal, the researchers could tell this event involved less mass than the previously observed black hole collisions.
“When we detected this event, my feeling was, wow, we have hit the motherlode,” said Laura Cadonati, an astrophysicist at the Georgia Institute of Technology and Ligo representative.
Just 1.7 seconds after the initial gravitational wave detection, Nasa’s Fermi space telescope registered a brief flash of gamma radiation coming from the constellation Hydra. Half an hour later, McEnery, the telescope’s project scientist, got an email from a colleague with the subject line, “WAKE UP”.
“It said, ‘This gamma ray burst has an interesting friend … Buckle up,’” McEnery recalled.
Gamma ray bursts are the most energetic forms of light in the cosmos. Scientists had long predicted that a short burst would be associated with a neutron star merger. That violent collision shoots jets of radioactive matter into space, as though someone had smashed their palm on a tube of toothpaste with holes at both ends. “We were beside ourselves,” McEnery said. Scientists raced to find the signal’s source before it vanished from the always expanding universe. “It is the classic challenge of finding a needle in a haystack, with the added complication that the needle is fading away and the haystack is moving,” said astrophysicist Marcelle Soares-Santos of Brandeis University in Massachusetts.
Gravitational waves travel at light speed. “Einstein predicts that gravity and photons move at the same speed … and [the signals] arrived within two seconds of each other, dramatically confirming that Einstein’s prediction is right,” McEnery said at a news conference last week.
Meanwhile, trigger alerts had gone out to Ligo collaborators at dozens of observatories around the globe. Ligo gave astronomers a narrow map of the
The detection involved thousands of researchers working at more than 70 labs and telescopes
sky to hunt for cosmic violence. “It was critical to know where to look,” said Edo Berger of Harvard University’s Center for Astrophysics. “If we were just searching blindly across the whole sky I don’t think we would have seen it.”
At Penn State University, phones began buzzing during a science operations team meeting for Nasa’s Swift satellite. From low Earth orbit, the Swift satellite cycled through 750 points in the sky until it detected “a vast avalanche of data” in the form of ultraviolet rays coming from the neutron star merger. They were just in time: the UV emission disappeared in less than 24 hours.
Ryan Foley, an astronomer at the University of California at Santa Cruz, was walking around an amusement park when he got the urgent text from one of his collaborators. He abandoned his partner in front of the carousel, jumped on a bike and pedalled back to his office.
He and his colleagues stayed up all night, first waiting for the sun to set on their telescope in Chile, then sorting through the telescope’s images in search of a “transient” – a new object in the sky.
In the ninth image, postdoctoral researcher Charlie Kilpatrick saw it: a tiny new dot beside a galaxy known as NGC 4993, 130m light years away.
He notified the group through the messaging service Slack: “@foley found something; sending you a screenshot”.
Foley marvelled at Kilpatrick’s measured tone in those messages. “Charlie is the first person, as far as we know, the first human to have ever seen optical photons from a gravitational wave event,” he said.
The event was named for the telescope that found it: Swope Supernova Survey 2017a.
Researchers collected data from the kilonova in every part of the electromagnetic spectrum. In the early hours the explosion appeared blue and featureless – the light signature of a very young, very hot new celestial body. But unlike supernovas, which can linger in the sky for months, the explosion turned red and faded. By separating light from the collision into its component parts, scientists could distinguish the signals of heavy elements like silver and gold.
For millennia the two dead stars circled each other approaching the speed of light, shaking off gravitational waves, which in turn pulled them closer together. When the husks smashed together, dinosaurs walked the planet. The shock wave from the collision finally reached Earth in August.
Scientists don’t know what happened in the wake of the explosion. Neutron stars are too faint to be seen from so far away, so researchers can’t tell if the merger produced one large neutron star, or if the bodies collapsed to form a black hole.
But after two months of analysis, the collaborators were ready to inform the world about what they have so far. Their results were announced last week in more than a dozen papers in the journals Nature, Science and the Astrophysical Journal Letters.
This kilonova was so bright that it could have been observed even by amateurs with tiny telescopes. In the future, Ligo will alert the whole world to potential detectors, allowing citizen scientists to join in the global search.
France Córdova, director of the National Science Foundation, which funds Ligo, compared traditional, visual astronomy to a silent film. The earliest gravitational wave detections added sound, but they were little more than strange noises echoing in the dark, she said. “We couldn’t pinpoint the location of the source.”
Now, for the first time, the soundtrack of the cosmos has synced up with what scientists can see.
“It’s really a triumph of science,” Foley said. “We as a civilisation have essentially been confined to the Earth, and almost all the information we’ve ever received from the universe has been through light. Yet we were able to predict … things as extreme as two neutron stars colliding when even the idea of neutron stars is incredible.”
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