How Australian scientists recorded ripples in space and time from a neutron star collision

An artists’s impression of two netron stars colliding. Image: LSC Sonoma State University / Aurore Simonett

Scientists have for the first time measured via gravitational waves the violent death of two neutron stars and seen the subsequent fireball.

The observation of ripples in space and time by an international team of astronomers, including dozens from Australia, comes less than a month after the discovery of gravitational waves won the 2017 Nobel Prize in Physics.

Scientists have described the findings as “astonishing”, one of the “biggest astronomical discoveries of the century so far”, and an “incredible feat in technology” some 40 years after gravity waves from black holes were first detected.

The explosion, produced by a pair of neutron stars merging, took place in galaxy NGC 4993, about 130 million light-years away.

The first follow-up detection was optical, about 11 hours after the event, and was detected by a number of groups worldwide. X-ray emissions were picked up nine days later and radiowaves after 15 days.

Neutron stars, the densest in the universe and measuring just 10km across, are found alone but also in pairs orbiting each other. As they circle each other, they radiate gravitational waves and their orbit shrinks.

Australian Telescope Compact Array. Image: David Smythe

Professor Susan Scott, chief investigator from the Research School of Physics and Engineering at The Australian National University, said: “We knew that eventually many of them must smash together in violent collisions but we had never seen it happen. The astronomers simply did not know where to point their telescopes at the right time.”

The international team detected the neutron star collision on August 17, alerting astronomers around the world to the likely existence of signals such as light, gamma rays and radio waves from the same event.

“This discovery of neutron stars colliding is just the beginning. We want to one day look back to the beginning of time — just after the Big Bang, which we can’t do with light,” says Professor Scott.

“This is the first time that the collision of two neutron stars has been detected, and this is the closest and most precisely located gravitational wave signal we’ve received. It is also the loudest gravitational wave signal we’ve detected.”

An explanation of the event:

Professor Scott says neutron star mergers are likely to be where much of the universe’s heavy metals such as gold, platinum and uranium are produced.

“With this discovery we have the opportunity to learn so much more about neutron stars, which have been quite a mystery to us,” she says.

“Unlike black holes, neutron star collisions emit other signals such as gamma rays, light and radio waves so astronomers around the world were able to observe the event through telescopes. This is an amazing time to be a scientist.”

ANU astronomer Dr Christian Wolf says his team used the SkyMapper and 2.3-metre telescopes at the ANU Siding Spring Observatory as part of the search for other signals from the neutron star collision.

“We saw the light from a fireball blasting out from the neutron star collision into space in the hours and days afterwards,” says Wolf.

“SkyMapper was the first telescope to report the colour of the fireball, which indicates the temperature of the fireball was about 6,000 degrees Celsius — roughly the surface temperature of the Sun.”

University of Sydney Associate Professor Tara Murphy, who leads the radio astronomy follow-up in Australia, says she was in the US with colleague David Kaplan when they saw the gravitational wave announcement come through on the private email list of the Advanced Laser Interferometer Gravitational-Wave Observatory (LIGO).

“We immediately rang our team in Australia and told them to get onto the CSIRO telescope as soon as possible, then started planning our observations,” she says.

“We were lucky in a sense in that it was perfect timing but you have to be at the top of your game to play in this space. It is intense, time-critical science.”

She explains in this video clip:

The team used the CSIRO’s Australia Telescope Compact Array to monitor the gravitational wave event for more than 40 hours over several weeks.

Professor Matthew Bailes, Director of The ARC Centre of Excellence for Gravitational Waves (OzGrav) and at Swinburne University of Technology, says: “This was the first time that any cosmic event was observed through both light it emitted and the gravitational ripples it caused in the fabric of space-time.

“Never before have we seen where in the universe gravitational waves came from; the subsequent avalanche of science was virtually unparalleled in modern astrophysics.”

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