Gravitational wave discovery from a huge cosmic collision leaves astronomers wondering what hit them

For only the second time in history, scientists have used the Laser Interferometer Gravitational-Wave Observatory (LIGO) instrument to pinpoint a violent collision of neutron stars, where only two super-dense remnants now exist. Unlike earlier instances where the signal was quickly followed-up, using other telescopes to pinpoint the source of the signal, the collision wasn't recorded by any visual telescopes.

What scientists did manage to record was a ripple in space-time fabric (gravitational wave) experienced on Earth, which suggests the origin are two massive entities that don't fit into the models that astronomers have developed for neutron stars – they're much bigger.

An artist's illustration of merging neutron stars. The rippling space-time grid represents gravitational waves that travel out from the collision, while the narrow beams show the bursts of gamma rays that are shot out just seconds after the gravitational waves. Image: NSf/LIGO/Sonoma State Uni/A Simonnet

In 2017, LIGO made a historic discovery when it detected a pair of neutron stars for the first time — enormous remnants left behind by massive star when they die. Locked in a spiral dance around each other, the neutron stars eventually merge, and the heavy objects smashing into each other this way causes a ripple in the fabric of space-time to spread out from the collision, and travel far and wide in the galaxy. LIGO was designed to pick up on these super-faint signals from the universe.

LIGO is currently in its third observation run, which is also during which time it picked up on the unusual neutron star collision, on 25 April 2019. The LIGO team reckons the neutron star is 3.4 times the mass of our Sun. So far, our telescopes are yet to catch a neutron star pair with a combined mass greater than 2.9 times that Sun's mass.

Researchers aren't ruling out the possibility that the entities are two light black holes and not massive neutron stars, according to Katerina Chatziioannou, an astronomer at the Flatiron Institute in New York City. Even so, black holes that small haven't been observed by astronomers either.

Two black holes merging in a massive collision [artist's impression] Image: SXS

"This is clearly heavier than any other pair of neutron stars ever observed," said at a press conference on 6 January at the 235th meeting of the American Astronomical Society in Honolulu.

After LIGO's 2017 discovery of merging neutron stars, astronomers also caught an accompanying burst of gamma-ray light, pointing to the exact source – an old galaxy 130 million light-years from Earth. For the current burst, so far, no such flash has been pick up on by astronomers. Since the signal was picked up, theoretical astrophysicists are considering many explanations for the lack of a light signal – a giant black hole was created quickly enough in the collision that it pulled in any outgoing light, or that the jet of energy was oriented away from our solar system when the collision took place, Chatziioannou said.

Astronomers will continue to study the event, as well as subsequent gravitational wave occurrences. In a few weeks, a new detector is expected to come online in Japan, helping scientists detect and pinpoint even more gravitational waves.

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