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'Kilonova' detection is astronomical gold mine, here's why

Scott Sutherland
Meteorologist/Science Writer

Monday, October 16, 2017, 12:13 PM - For the first time ever, astronomers have spotted a 'kilonova', an elusive, previously only theoretical stellar explosion, and this discovery is not only important for the science of gravitational waves, but may also solve another puzzle of the universe.

130 million of years ago, in another galaxy, two neutron stars orbiting each other came so close together that they merged. The titanic forces produced by this merger set off ripples across space-time, known as gravitational waves, and very shortly thereafter, it also caused an immense explosion, 1,000 times brighter than a typical stellar nova - a kilonova.

On August 17, 2017, the gravitational waves from this merger were picked up by both the LIGO and VIRGO detectors. This was the fifth gravitational wave detection made, known as GW170817, but it was the first detected from a pair of neutron stars, and it is now the first ever where astronomers also spotted the kilonova explosion that went with it.


Nova, supernova, kilonova?

Two types of explosions associated with stars - novae and supernovae - are well known and have been observed numerous times by astronomers. A nova occurs when a white dwarf - the 'dead' remnant of a Sun-like star - pulls matter off a larger companion star, and the intense gravity at the surface of the white dwarf causes that stolen matter to fuse, ignite and expand out into space. The pair brightens significantly from this eruption, and they can appear as a new star in our night sky, thus inspiring the name 'nova' (which means "new"). A supernova is a much more energetic explosion, a million times brighter than a nova. It can be caused by the collapse of a massive star, into either a neutron star or a black hole. It can also happen in a similar way to a nova, except that rather than only the stolen matter fusing, nearly the entire mass of the white dwarf undergoes fusion, resulting in a colossal explosion that blows the entire remnant apart.

Over the past 30 years, theoretical physicists have investigated how other stellar remnants - a pair of neutron stars, a pair of black holes, or a paired neutron star and black hole - could interact and merge. This work gave us the ideas behind gravitational waves, and what they would look like if we detected them, but it also gave us an idea of what an explosion from the merger of two neutron stars should look like - a 'kilonova', or what some call a 'macronova'.

These kilonova explosions are thought to be responsible for dispersing heavy elements, such as gold and platinum, throughout the Universe.

Up until now, astronomers had not actually seen one of these kilonovas, though. With the detection of the burst of light from GW170817, they appear to have the first.

"The data we have so far are an amazingly close match to theory," said Stefano Covino, the lead author of a Nature Astronomy paper detailing the detection of the kilonova, according to ESO News. "It is a triumph for the theorists, a confirmation that the LIGO–VIRGO events are absolutely real, and an achievement for ESO to have gathered such an astonishing data set on the kilonova."

With this detection, astronomers were able to pinpoint the source, in the galaxy NGC 4993, some 130 million light years away. That makes this first kilonova not only the first confirmed light seen from a gravitational wave event, but it also confirms GW170817 as being the closest gravitational wave event so far, and also the closest gamma ray burst source ever.

This mosaic of images, taken by the VISTA infrared survey telescope at ESO's Paranal Observatory in Chile, shows how the kilonova in NGC 4993 - the bright dot to the upper left of the galaxy's center - brightened, became much redder in colour and then faded in the weeks after it was first detected, on August 17, 2017. Credit: ESO/N.R. Tanvir, A.J. Levan and the VIN-ROUGE collaboration

Another mystery solved?

Along with those discovery milestones, we may be able to add another.

Over the years, NASA’s Fermi Gamma-ray Space Telescope and the ESA’s INTErnational Gamma Ray Astrophysics Laboratory (INTEGRAL), have detected a number of very short bursts of gamma rays, the most energetic rays we find. These have lasted anywhere from milliseconds up to two seconds long, and have been observed all over the sky. It was speculated that these may be caused by the merger of two neutron stars, however their source has remained a mystery, since there has been no way to confirm that hypothesis. Regardless of the many short gamma ray bursts detected, astronomers lacked some kind of supporting evidence to link them to a source.

With this detection, it would appear that they now have that supporting evidence. Seeing the gravitational waves from a neutron star merger show up at the same time, and from the same location, as one of these short gamma ray bursts shows that they are linked.

Sources: ESO | NASA

Watch Below: What is a Gravitational Wave and what causes them?

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