NASA Missions Catch First Light from a Gravitational-Wave Event

For the first time, NASA scientists have detected light tied to a gravitational-wave event, thanks to two merging neutron stars in the galaxy NGC 4993, located about 130 million light-years from Earth in the constellation Hydra.

Swift’s Ultraviolet/Optical Telescope imaged the kilonova produced by merging neutron stars in the galaxy NGC 4993 (box). Image Credits: NASA/Swift[/CREDIT]

Shortly after 8:41 a.m. EDT on Aug. 17, NASA's Fermi Gamma-ray Space Telescope picked up a pulse of high-energy light from a powerful explosion, which was immediately reported to astronomers around the globe as a short gamma-ray burst.

The scientists at the National Science Foundation’s Laser Interferometer Gravitational-wave Observatory (LIGO) detected gravitational waves dubbed GW170817 from a pair of smashing stars tied to the gamma-ray burst, encouraging astronomers to look for the aftermath of the explosion.

NASA's Swift, Hubble, Chandra and Spitzer missions, along with dozens of ground-based observatories, including the NASA-funded Pan-STARRS survey, later captured the fading glow of the blast's expanding debris.

Neutron stars are the crushed, leftover cores of massive stars that previously exploded as supernovas long ago. The merging stars likely had masses between 10 and 60 percent greater than that of our Sun, but they were no wider than Washington, D.C.

The pair whirled around each other hundreds of times a second, producing gravitational waves at the same frequency. As they drew closer and orbited faster, the stars eventually broke apart and merged, producing both a gamma-ray burst and a rarely seen flare-up called a "kilonova."

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. LIGO tells us there was a merger of compact objects, and Fermi tells us there was a short gamma-ray burst.

Within hours of the initial Fermi detection, LIGO and the Virgo detector at the European Gravitational Observatory near Pisa, Italy, greatly refined the event's position in the sky with additional analysis of gravitational wave data. Ground-based observatories then quickly located a new optical and infrared source -- the kilonova -- in NGC 4993.

To Fermi, this appeared to be a typical short gamma-ray burst, but it occurred less than one-tenth as far away as any other short burst with a known distance, making it among the faintest known. Astronomers are still trying to figure out why this burst is so odd, and how this event relates to the more luminous gamma-ray bursts seen at much greater distances.

When Swift mission turned to the galaxy shortly after Fermi’s gamma-ray burst detection, it found a bright and quickly fading ultraviolet (UV) source.

This was produced by the short-lived disk of debris that powered the gamma-ray burst.

Over time, material hurled out by the jet slows and widens as it sweeps up and heats interstellar material, producing so-called afterglow emission that includes X-rays.

But the spacecraft saw no X-rays -- a surprise for an event that produced higher-energy gamma rays.

NASA’s Chandra X-ray Observatory clearly detected X-rays nine days after the source was discovered. Scientists think the delay was a result of our viewing angle, and it took time for the jet directed toward Earth to expand into our line of sight. (NASA)

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