Something big went boom in a distant galaxy. It's wasn't a nova. It's wasn't a supernova. It was a kilonova, and it burst forth with enough energy that four different telescopes monitoring virtually the entire spectrum of energy picked it up. And before astronomers saw any visual evidence of this cataclysmic collision, their instruments picked up the movement of gravitational waves sending ripples through the fabric of space-time.

In research published in three different journals today (Nature, Nature Astronomy and Astrophysical Journal Letters), hundreds of physicists and collaborators outline a first-of-its-kind observation: the elusive neutron star merger.

"Because we've seen this light show that accompany the gravitational wave event we believe at least one of the objects had to be a neutron star," says Nergis Mavalvala, an MIT professor and collaborator with the Laser Interferometer Gravitational-Wave Observatory (LIGO). The team believes both objects were neutron stars, "but as scientists we can't say for certain" that the heavier object wasn't a small black hole.

Neutron stars are the dense cores of stars that have previously gone supernova and shed their outer material. If the remaining core of the star is less than two-and-a-half times the mass of the sun, it becomes a six-mile diameter ball of dense, all-neutron matter. Any more massive, and the star will collapse into a black hole. Neutron stars are the second densest known objects in the universe after black holes, and both form under similar circumstances.

"The uncertainty comes from the fact that there's no hard boundaries between what mass a neutron star should have and what mass a black hole should have," says Mavalvala.

The merger as seen by ESO's GROND telescope. ESO/S. Smartt & T.-W. Chen

When the explosion from a neutron star merger occurred in the galaxy NGC 4993, which is 130 million light-years away, it sent physical ripples through the fabric of space-time. These gravitational waves were strong enough that the two LIGO observatories and the European sister station, Virgo, all picked up the signals. Seconds later, the Fermi Gamma-ray Space Telescope saw a bright flash called a short gamma ray burst that lasted two seconds. Then the fireworks of the explosion set off, viewed by several ground-based observatories.

This is the fourth gravitational wave event documented by LIGO in the last two years, although the newest cosmic event is unique. The previous three detections of gravitational waves came from black hole mergers, while this neutron star merger involved much smaller objects and had an optical component as researchers detected the gamma ray burst and light from the the kilonova explosion moments after the gravitational waves.

The collaborative effort between LIGO, Virgo, and multiple additional observatories demonstrates the power of these instruments to find smaller and smaller gravitational events. The Virgo interferometer in Europe was critical to pinpoint the origin of the merger because it's oriented differently from LIGO, allowing the gravitational waves to be traced to the source. If more neutron star mergers occur, collaboration between LIGO and Virgo can allow ground observatories to immediately point their telescopes to the event epicenter like during the NGC 4993 merger.

The new detection of gravitational waves also serves as a benchmark in a new era of astronomy where violent but nearly-invisible cataclysms can be "felt" as they rip through the fabric of space itself.

The event is technically still in progress as researchers continue to measure the incoming gravitational waves here on Earth. The LIGO and Virgo teams don't quite know what is being created at at the center of the cataclysm—it could be a larger neutron star, or the accumulated mass may be enough to collapse into a black hole. Mavalvala says it's hard to even speculate right now because the neutron star merger is the first such event ever observed.

"We're still culling the data," Mavalvala says. "It's just too early to say, and I'm not holding back."

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