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Scientists Spot the Spark From Ancient Collision of Neutron Stars

The chirp and flash from the event offers clues to the origin of Earth’s precious metals

An illustration of two neutron stars merging, ejecting gamma ray streams and clouds of matter that produce heavy elements and light (National Science Foundation/LIGO/Sonoma State University/A. Simonnet)
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A global team of astronomers has detected the bright spark of two neutron stars colliding, shedding light on the previously unknown origins of some of the universe's heavy elements.

On August 17, scientists operating the Laser Interferometer Gravitational-wave Observatory (LIGO) detected another round of gravitational waves. Researchers have seen such ripples four times before, but this latest sighting differed from the rest: Astronomers not only heard the “chirp” of the ancient collision, they saw a flash of light.

“Imagine that gravitational waves are like thunder. We’ve heard this thunder before, but this is the first time we’ve also been able to see the lightning that goes with it,” Philip Cowperthwaite, researcher at the Harvard-Smithsonian center for Astrophysics, says in a press release.

Predicted by Albert Einstein in 1916, and first spotted by scientists in 2015, these distortions in the fabric of space-time come from the violent movements or collisions of celestial objects. But scientists haven't yet been able to identify the objects causing these distortions. In September, researchers announced that they were narrowing in on the source of the waves using triangulation between two LIGO observatories in the U.S. and the European Virgo observatory.

Even so, until now researchers have largely been left in the dark about where the collisions occur. Previous chirps were thought to come from colliding black holes, which as their name suggests, emit little to no light, making them nearly impossible to spot in the night sky.

But this time was different.

Immediately following the chirp of this latest gravitational wave detection, NASA’s Fermi Space telescope recorded a flash of gamma radiation. So the researchers began sending alerts out to collaborators around the world of the exciting opportunity; perhaps they could image the collision.

Graduate student Charlie Kilpatrick, operating a telescope in Chile, was the first to spot it: a tiny speck of light next to the galaxy NGC 4993, which lies some 130 million light-years from Earth. Teams operating 70 telescopes on every continent (Antarctica included) trained their sights on this region of the sky, scrutinizing it in a range of wavelengths from X-rays to radio waves in search of the source for these cosmic ripples.

Based on their observations, the scientists believe this latest waves came from the violent merger of two neutron stars—the dense, dying remnants of massive stars after they undergo a supernova. Scientists indirectly observed the debris from the collision moving at speeds so rapid that models suggest they could only be achieved if two of these celestial bodies collided. These two now-famous neutron stars likely formed roughly 11 billion years ago, according to the astronomer's analysis of their galaxy, and have slowly been drifting toward each other ever since.

"This is the first time we can hear the death spiral of two neutron stars, and also see the fireworks that came from their merger," Vicky Kalogera, director of Northwestern University's Center for Interdisciplinary Exploration and Research in Astrophysics and a leader in the LIGO Scientific Collaboration, said in a press conference today about the find.

The light show from the collision held even more interesting clues to untangle. Researchers have long speculated that the heavier elements of the universe, such as gold or platinum, arose from the explosions, or kilonovas, produced by neutron star mergers. When observing the light coming from NGC 4993, astronomers saw telltale evidence of radiation produced by the matter from the kilonova cooling into heavy elements. A single kilonova can produce an entire Earth's worth of these rare elements, according to the researchers.

The initial results of this detection were published today in the journal Physical Review Letters, with more studies to come in the near future.

The astronomers involved in this detection see a bright, sonorous future for so-called "multi-messenger" astronomy, or using both gravitational waves and old-fashioned light to study the same events and objects in the sky. As Virgo spokesman Jo van den Brand said at the press conference: "I think this is a demonstration of what mankind can achieve if we put our minds to it and if we collaborate."

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