In a highly anticipated first, scientists said they’ve detected the collision of two neutron stars and confirmed that these cataclysmic events are indeed a source of gold, platinum and other heavy elements in the universe.
The powerful smash-up produced gravitational waves that were picked up in mid-August by specialized observatories in the United States and Italy and a burst of gamma rays that was seen a few seconds later. News of the observations spread quickly among astronomers and other researchers, setting of a flurry of activity as they raced to find the stars in the sky and capture the event using more traditional telescopes.
Researchers at the University of Maryland played key roles in detecting and analyzing the event’s signals — both the gravitational waves and across the spectrum of light.
By studying the gravitational waves, gamma rays, X-rays, ultraviolet light, infrared, radio waves and visible light from such a single event, the scientists said they have embarked on a new era astronomy — one that promises a far deeper understanding of some of the most powerful and elusive phenomena in the cosmos. Researchers involved in the work say they’re living through a shift that comes once every few generations.
“It’s like being in a dream,” said Alessandra Buonanno, a physics professor at the University of Maryland, College Park. “I feel so lucky that I’m doing research and I'm working in a field that went through this transformation. It was not at all guaranteed when I entered the field that this would have happened in my lifetime.”
The findings, described in a suite of papers published Monday, mark the first time the larger astronomical community has been able to study a gravitational-wave event.
Previously, the LIGO observatories and its European partner Virgo have picked up only collisions between black holes — powerful events that can’t be detected with telescopes because not even light can escape a black hole’s powerful gravitational pull.
Scientists with the LIGO-Virgo collaboration had been itching to find a collision between two neutron stars because it would produce both gravitational waves and electromagnetic waves, including light.
Eleonora Troja, an astrophysicist at NASA’s Goddard Space Flight Center Center and the University of Maryland, was vacationing in Italy when she received an email saying the detectors had picked up something new. At first, she didn’t pay much attention figuring it was just another pair of black holes, but she soon realized it was something else.
“After a few minutes my phone started to ring like it was burning,” she said. “It was clear it was a watershed moment in astronomy.”
Working in her parents’ living room, Troja scrambled to get access to telescopes, commandeering the Hubble Space Telescope and others to begin looking at the event.
Neutron stars are the corpses of massive stars that have burned out in supernova explosions. While they’re not that big, they’re incredibly dense, packing a sun’s worth of mass into the size of a city. A teaspoon of neutron-star stuff weighs around a billion or so tons.
The cosmic crash described in numerous scientific papers Monday occurred about 130 million light-years away in the constellation Hydra, said David Reitze, executive director of the LIGO Laboratory at Caltech in Pasadena, Calif. After a long dance toward each other, two neutron stars — one somewhere around 1.1 solar masses, the other weighing in the neighborhood of 1.6 suns — finally collided, converting some of their combined mass into gravitational waves.
These waves are ripples created by objects as they accelerate or decelerate, rather like the wake made by a boat moving through the water. Albert Einstein predicted their existence 101 years ago as part of his general theory of relativity, and LIGO scientists won a Nobel Prize this month for detecting gravitational waves and proving Einstein correct.
Buonanno’s work has focused on using the theory to design models of the waves from different kinds of events. Those models are vital for picking out the faint signal that registers on the gravitational wave detectors and helped confirm that the waves detected Aug. 17 were from colliding neutron stars.
Buonanno was also on vacation and, like Troja, not about to disrupt it over the discovery of more colliding black holes. She told her postdoctoral students to only email her personal account if they saw what they thought were neutron stars.
That was the message that came on Aug. 17, after LIGO’s twin detectors in Hanford, Wash., and Livingston, La., measured a powerful gravitational “chirp.”
This signal, dubbed GW170817, looked very different from the waves produced by colliding black holes, Reitze said. Since neutron stars are less massive than black holes, a doomed pair takes longer to complete its final death spiral and packs many more waves into a single event.
Two seconds after the gravitational wave was detected, NASA’s Fermi Gamma-ray Space Telescope picked up a powerful flash of high-energy gamma rays. (The gamma rays are produced after the gravitational waves, but Fermi was the first to send out an alert.) Both Fermi as well as the LIGO and Virgo detectors were able to identify a patch of sky that was the likely source of their event — and those two areas overlapped.
Within hours, astronomers were training their telescopes on that promising region, looking for X-rays, ultraviolet waves, optical light, infrared light and radio waves. Each of these bands of the electromagnetic spectrum yields different kinds of information about the source, allowing the researchers to study this neutron-star collision in unprecedented detail.
Among their discoveries: Infrared cameras found signs that heavy elements such as gold, platinum and neodymium had been produced by this powerful event. While nickel, copper, iron and other elements can be produced by supernovae, scientists have long suspected that many elements heavier than iron are born from of the collision between neutron stars.
“They’re really cosmic foundries for heavy elements like gold, platinum, uranium,” Reitze said. “That’s pretty amazing.”
In the hours and days following the first detection of the collision, Troja focused on gathering data visible to the various telescopes in orbit and around the world.
What she and her team saw in the infrared light told them the event created a stunning amount of gold and other heavy elements — a mass several hundred times that of Earth’s, according to their research published Monday in the journal Nature.
That would make neutron star collisions the primary source of the universe’s heavy elements.
Troja also was confounded by what she couldn’t find at first — X-rays. They showed up after 9 days — seemingly from a new black hole produced in the aftermath of the neutron stars’ collision, Troja said.
"We had absolutely no clue what was going on,” she said. “Every day was bringing out new information, new discovery. We had to adjust our knowledge based on the new data.”
She described the X-rays as a theoretical phenomenon known as an orphan afterglow. A transient byproduct of gamma ray bursts, an orphan afterglow had not been confirmed by observation before though astronomers have been seeking one for nearly two decades.
Buonanno, who also serves as director at Max Planck Institute for Gravitational Physics in Germany, said the new insights came in rapid succession, contributing significantly to several fields in the space of just a few days. The data gathered has shed more light on the composition of neutron stars and the expansion of the universe.
Buonanno compared it to dominoes falling: “One thing happened after the other in a way that people predicted for many years, but we didn't observe it until now.”