Gravitational Waves Point to First Light from Merging Neutron Stars

Artist's concept of the explosive collision of two neutron stars, detected for the first time using gravitational waves. | Illustration by Robin Dienel courtesy of the Carnegie Institution for Science

Hundreds of scientists around the world sprang into action after the detection of gravitational waves last August, scouring the sky for the source of the waves. What they saw, for the first time, was the massive collision and merger of two extremely dense neutron stars.

In seven papers published online October 16 in Science, researchers reveal the valuable data collected in the critical hours following the stars' merger. The event represents the first detection of light from a gravitational wave source.

Gravitational waves are ripples in time-space that only occur under a handful of extreme cosmic conditions. The first observation of gravitational waves occurred in 2015, a feat that was recognized this October when three scientists won the 2017 Nobel Prize in Physics for their work on gravitational waves and the LIGO observatory that detected the waves.

All of the gravitational waves detected since 2015 have been produced by the merger of two black holes. However, the merging of black holes is not expected to produce any electromagnetic radiation, rendering conventional telescopes unable to detect the event.

Neutron stars are the collapsed cores of exploded supernovae, packed so densely that a single teaspoon of neutron star material would weigh one billion tons on Earth. The collision and merger of two neutron stars, sometimes called a kilonova, has long been expected to produce an energetic explosion and a plume of radioactive material that can be detected. Yet, direct observation of such an event has eluded astronomers to date.

That changed when two colliding neutron stars sent ripples through time-space that reached Earth on August 17. Scientists dubbed the event GW170817, denoting the date of the detection.

The four previous detections of gravitational waves had lasted at most a few seconds, with gravitational waves rippling at frequencies of tens of cycles per second. The detection in August, however, lasted for 100 seconds at frequencies up to thousands of cycles per second. Just two seconds after the waves were detected, the Fermi Gamma-Ray Space Telescope recorded a brief flash of gamma rays, hinting that the event was different than a black hole merger and prompting teams of scientists around the world to jump into action.

David Coulter of the University of California, Santa Cruz and his colleagues played an important role in pinpointing the location of the kilonova. The team used the Swope 1-meter telescope in Chile to hunt for light emitted by the merger, searching an area of the sky from which the gravitational waves were thought to have come from. Using a catalog of known nearby galaxies, they created a prioritized list of likely locations and began snapping images of them.

In the ninth image, they spotted a new source and identified the location of the event, within a galaxy called NGC 4993, about 130 million light years away. As the first team to locate the source, they dubbed it Swope Supernova Survey 2017a (SSS17a). They managed to locate the event 10.9 hours after GW170817 was first detected.

The Swope 1-meter telescope at Las Campanas Observatory in Chile, where light from the neutron star collision was first detected. | Yuri Beletsky

Immediately after SSS17a was identified, astronomers around the world focused their telescopes on region, collecting invaluable X-ray, ultraviolet, optical, infrared and radio data.

"There was a lot going on," said Josh Simon, an astronomer at Carnegie Mellon University whose team began analyzing the properties of SSS17a as soon as it was located. "It was probably the busiest and most stressful start to a night that I've had."

"It might have just happened to be a supernova that was there …. But this didn't look anything like a random supernova spectrum, which would have lots of different lines and bumps and features from different elements that we can see," explained Maria Drout, also of Carnegie University. "This was really something different. And we could tell right away."

Data from the seven studies published in Science suggest that, following the merger, matter comprising about 5% of the weight of the sun was spewed forth at an estimated 30% of the speed of light — substantially faster than a supernova. What's more, the reaction appears to have produced a class of heavy elements called lanthanides.

Scientists have theorized that kilonovae create such elements of mysterious origins, but until now they were unable to confirm that theory. Based on the data collected from SSS17a and from estimates of neutron stars throughout the Universe, scientists believe that kilonovae are a key source of heavy elements throughout the cosmos.

SSS17a was hotter in its early stages than theoretical models of kilonovae have predicted. However, the source quickly faded and changed from a blue to a red color — a sign that the material was expanding rapidly and cooling as it went.

Mansi Kasliwal and colleagues at the Observatories of the Carnegie Institution collected observations at X-ray, ultraviolet, optical, infrared and radio wavelengths to produce a detailed theoretical model of the merger. Using data from 24 telescopes on seven continents, they reconstructed the total energy emitted by the event at each stage, then sought to explain the observations at all wavelengths.

In their preferred model, a jet of material is produced during the merger that expands at close to the speed of light, but is directed away from our line of sight on Earth. Instead, we see emission from a 'cocoon' of shocked material surrounding the jet, which expands over a wider angle, according to the model. After confirming this model with detailed simulations, the researchers predicted that about 30% of future neutron star mergers will produce bright gamma rays that will reach Earth.

At an October 16 press conference in Washington, D.C., David Shoemaker of MIT and spokesperson for the LIGO team said that they have shut down the gravitational wave detector for now, so that scientists can fine-tune the detector and make it even more sensitive. The LIGO team will be collecting substantially more data in the future, he said, and will need to coordinate with other researchers to better handle the volume of data, as more of these events are detected and analyzed. "It's a good dilemma to have," Shoemaker said.

At the press conference, Vicky Kalogera of Northwestern University described how these new data confirm much of what Albert Einstein predicted about gravitational waves; for example that the gamma rays and gravitational waves from such an event will travel together at the speed of light. The merging of the two neutron stars resulted in gamma rays and gravitational waves that reached Earth just seconds apart. Einstein also predicted that kilonovae would produce elements that are heavier than iron, which the new data also supports.

"It's amazing to think in one day, all these predictions were confirmed," said Kalogera. "We solved a lot of mysteries, but we also opened up a new set of questions. We're hoping new observations … are going to answer a lot of those questions. The future of astronomy is definitely bright."