An artist’s impression shows two tiny but very dense neutron stars at the point they merge and explode as a kilonova. Such a very rare event is expected to produce both gravitational waves and a short gamma-ray burst, both of which were observed on August 17, 2017. Photo by L. Calçada and M. Kornmesser/via ESO

Neutron star collision offers new source of gravitational waves

Science

An artist's impression shows two tiny but very dense neutron stars at the point they merge and explode as a kilonova. Such a very rare event is expected to produce both gravitational waves and a short gamma-ray burst, both of which were observed on August 17, 2017. Photo by L. Calçada and M. Kornmesser/via ESO

Gravitational waves are back. And this time, they're not traveling alone. In the first four detections of these astronomical phenomena, gravitational waves emanated from merging binary black holes—a source that puts off no light.

On Monday, Astronomers from LIGO, the Laser Interferometer Gravitational-Wave Observatory, and the Virgo detector in Italy announced in a press conference that they discovered a collision of neutron stars that released both a stream of gravity waves and a flash of light. These findings–published in a suite of Science and Nature papers–back decades-old theories, including one by Albert Einstein that gravitational waves travel at the speed of light.

"We're starting to see the whole universe in front of our eyes."

"It's a privilege to discover that neutron stars can also emit gravitational waves, especially so close to the original gravitational wave discovery's second anniversary," said Stefano Covino, a researcher at the National Institute for Astrophysics in Italy and lead author of a Nature Astronomy paper that details some of the announced findings. LIGO and Virgo identified the gravitational waves from the twin neutron stars on August 17. At about 130 million light years away, this is the nearest gravitational wave event detected so far.

Astronomers verified the source by the neutron stars' low masses. Black holes tend to start at around three to five times the mass of our sun. The new source ranged from 1.1 to 1.6 solar masses, which fit the bill for a neutron star.

This is the coalescence of two orbiting neutron stars. The left panel shows the matter of the neutron stars while the right panel shows how spacetime distorts near the collision. Image by Christopher W. Evans/via Georgia Tech

Though it doesn't seem massive, a neutron star is the collapsed core of a giant star after it goes supernova. They're packed with neutrons (hence the name) and are usually about 12 miles in diameter. The density of a neutron star is so great that scooping up a teaspoon of its matter would weigh more than a billion tons. So, neutron stars — like black holes — were considered by astronomers to be massive enough to interact with the universe; thus, causing gravitational ripples in the curvature of spacetime.

Neutron star mergers were what scientists initially expected to find with LIGO, explained astrophysicist France Córdova, director of the National Science Foundation, which funds LIGO.

"We arguably know a lot more about neutron stars and can imagine them in binaries more than large black holes," Córdova said. "It was a real surprise when we first found a few black hole mergers that were farther away than this star merger."

However, LIGO and Virgo data show that neutron star collisions may be less common than expected, Vicky Kalogera, an astrophysicist at Northwestern University and with the LIGO collaboration, said at the press conference. In rough estimates, neutron star mergers may be happening between 30 to 500 times in the Milky Way over the course of millions of years, Kalogera said.

The way two massive objects become one is virtually the same, but for the neutron stars, it takes time. As the stars swirled around each other, they began losing angular momentum which slowly closed the gap between them.

What's left in the gamma ray afterglow are rare heavy elements, such as uranium, platinum and gold. Yes, the material in your jewelry and gadgets may have come from neutron star collisions.

Previous observations of black holes showed the merger happening in an audible "chirp" that lasted less than a second. Black holes quickly bubble together rather than crash. For the neutron stars, the gravitational wave chirp lasted about 100 seconds.

Two seconds after the waves passed through the Earth, NASA's Fermi Gamma-ray Space Telescope and the European Space Agency's gamma-ray observatory INTEGRAL spotted a surge of high-energy light after the cataclysmic collision. This finding meant the merger of stars caused a burst of short gamma-rays and then a kilonova, 1,000 times brighter and more violent than the average nova. What's left in the gamma ray afterglow are rare heavy elements, such as uranium, platinum and gold. Yes, the material in your jewelry and gadgets may have come from neutron star collisions.

"These kinds of explosions as the source for the universe's heavy elements have been theorized for a long time by people looking at how elements get synthesized in the collapse of stars or in neutron stars," Córdova said. "Now we have confirmed that this kind of explosion produces those. Next, scientists need to figure out how this all happens in these very energetic explosions."

After all these detections happened in a span of minutes, alerts sounded at 70 ground- and spaced-based observatories, who quickly guided their telescopes to the same swath of sky. The multinational collaborators watched for a variety of electromagnetic radiation—gamma rays, x-rays, ultraviolet, visible light, infrared and radio waves—to confirm LIGO and Virgo data in the next days and weeks.

David Reitze, executive director of the LIGO Laboratory, said these findings may usher in a new field called "multi-messenger astronomy."
"This is the first time the cosmos has provided us a 'talking movie,'" Reitze explained. "We're going from the era of silent movies to talking movies. In this case, the audio soundtrack comes from the chirp of the neutron stars as their spiraling together. The video is basically the light that we see from the collision."

Like the prediction that neutron stars would radiate gravitational waves, theorists had long hypothesized that these same colliding bodies could send out short gamma-ray bursts from a kilonova. This recent event finally confirms this suggestion and adds weight to Einstein's general theory of relativity. Detecting gamma rays and gravitational waves at the essentially same time confirms his prediction that gravitational waves travel at the speed of light.

"[These validations] show that we have the capacity to understand the universe. We're starting to see the whole universe in front of our eyes," Covino said.

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