In the galaxy NGC 4993, located approximately 130 million light-years from Earth, two neutron stars collided. And, for the first time, scientists detected the gravitational waves to prove it. They may have even solved the long-standing mystery about the origin of gold and platinum.

This week, astronomers announced the detection of high-energy light from the explosion: a gamma-ray burst and a rarely seen flare-up called a "kilonova."

The discovery comes over two years after the first observation of gravitational waves in September of 2015.

NASA's Spitzer Space Telescope has provisionally detected the faint afterglow of the explosive merger of two neutron stars in the galaxy NGC 4993. The event, labeled GW170817, was initially detected in gravitational waves and gamma rays. (Credit: NASA)

Shortly after 8:41 a.m. EDT on Aug. 17, NASA scientists, using the agency’s Fermi Gamma-ray Space Telescope, picked up the short gamma-ray burst. Additionally, scientists at the National Science Foundation-funded Laser Interferometer Gravitational-wave Observatory (LIGO) detected just what the facility name implies: gravitational waves, officially labeled GW170817.

The growing, glowing debris from the explosion was captured and confirmed by a combination of sources, including ground-based observatories, the Pan-STARRS survey, and NASA's Swift, Hubble, Chandra and Spitzer missions.

"The favored explanation for short gamma-ray bursts is that they're caused by a jet of debris moving near the speed of light produced in the merger of neutron stars or a neutron star and a black hole," said Eric Burns, a member of Fermi's Gamma-ray Burst Monitor team at NASA's Goddard Space Flight Center in Greenbelt, Maryland, in a NASA press release. "LIGO tells us there was a merger of compact objects, and Fermi tells us there was a short gamma-ray burst."

The two "neutron stars," leftover cores of exploded supernovas, orbited each other, producing gravitational waves at the same frequency. As the bodies drew closer, the stars eventually broke apart and merged, resulting in the kilonova and gamma-ray combination.

Scientists and astrophysicists believe neutron stars may be the universe's dominant source for many of the heaviest elements, including platinum and gold.

Within hours of the initial Fermi detection, LIGO and the Virgo detector at Italy's European Gravitational Observatory, pinpointed the event's position in the sky. Ground-based observatories then quickly discovered a new optical and infrared source — the kilonova — in NGC 4993.

On Aug. 22, NASA’s Hubble Space Telescope began imaging the kilonova and capturing its near-infrared spectrum, which revealed the motion and chemical properties of the expanding debris.

"This is extremely exciting science," said Paul Hertz, director of NASA’s Astrophysics Division at the agency’s headquarters in Washington. "Now, for the first time, we've seen light and gravitational waves produced by the same event.”

The detection of a gravitational-wave source’s light, said Hertz, has revealed details of the event that cannot be determined from gravitational waves alone.

Brad Cenko, Research Astrophysicist at the NASA Goddard Space Flight Center in Greenbelt, MD, shares Hertz’s excitement. Cemko spoke with Tech Briefs about yesterday’s NASA announcement – and what kinds of observations we can expect in the future.

Tech Briefs: What can the detection of gravitational waves tell us?

Brad Cenko: A lot! The gravitational wave detection that was reported yesterday tells us that, approximately 130 million years ago, two very dense objects, each weighing about 1.4 times our Sun, collided.

In particular, we call these objects neutron stars, since they are essentially a Manhattan-sized ball of neutrons, or uncharged subatomic particles. In addition to the existence of this collision, and its distance (130 million light years), the gravitational waves can tell us where on the sky (e.g., in which constellation) this collision occurred. This is absolutely critical in the search for light from this merger.

Tech Briefs: What makes the discovery an exciting one to you?

Cenko: Gravitational waves were detected from the merger of two neutron stars. This is the first time we’ve directly seen such a collision. Previous gravitational wave detections all involved black holes.

But what makes this really exciting is that we were able to see light — gamma-rays, X-rays, ultra-violet, optical, infrared, and radio waves — that were emitted from the explosion that resulted from this collision. This is the first time we’ve been able to see light associated with a gravitational wave source.

Tech Briefs: What can the light show us?

Cenko: The glowing debris from this explosion, for example, showed the characteristic features of heavy element formation, in particular elements much heavier than iron. We think that this system formed about 100 solid earths’ worth of gold and platinum, for instance — a veritable cosmic mine! In fact, such neutron star mergers, in the very distant past, may have been responsible for precious metals like gold and platinum on Earth.

Tech Briefs: What are the key technology features that enable the observations?

Cenko: First and foremost, the gravitational wave detectors are remarkable feats of technology and engineering. In order to detect gravitational waves, they search for changes in distance between two mirrors of less than 1/1000th of an atomic nucleus, over a distance of several miles!

Tech Briefs: Where do we go from here? What are NASA's short-term and long-term goals with these kinds of observations?

Cenko: The great thing is that this is really just the beginning! LIGO is shutting down for approximately one year to improve its sensitivity. When it comes back online, we should expect detections of binary neutron star collisions to become more and more routine.

And NASA’s entire suite of space telescopes, including Fermi, Swift, Chandra, and Hubble, will continue to search for light associated with these collisions. We’d like to know: Is this first one special, or is this how all neutron star mergers look? Why was the gamma-ray signal seen by Fermi relatively faint? Does each neutron star collision produce the same amount of gold and platinum?

This pairing of gravitational waves and light is sure to yield more surprises!

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