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Gravitational Waves Detected from Two Stars Colliding

Australian astronomers have confirmed the first-ever detection of gravitational waves produced by the collision of two neutron stars about 130 million light-years away in galaxy NGC 4993.

The explosion was detected by NASA’s Fermi gamma ray telescope about 2 seconds after the gravitational wave was detected by the US-based Laser Interferometer Gravitational-wave Observatory (LIGO) and Europe-based Virgo Collaboration.

Visible light and gamma-ray emissions allowed astronomers to pinpoint the location of the collision. The first follow-up detection was optical about 11 hours after the event, and was observed by a number of groups worldwide. X-ray emissions were detected 9 days later and radio waves after 15 days.

A/Prof Tara Murphy of The University of Sydney, who led the radio astronomy follow-up in Australia, said she was in the United States when the gravitational wave discovery was announced on LIGO’s private email list. “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 said. “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.”

The team used CSIRO’s Australia Telescope Compact Array to monitor the gravitational wave event for more than 40 hours over several weeks. The results are included in a Science paper, with follow-up observations by other international teams published in several science and astronomy journals simulateneously.

Dr Christian Wolf said his team at the Australian National University 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,” he said. “SkyMapper was the first telescope to report the colour of the fireball, which indicates the temperature of the fireball was about 6000°C – roughly the surface temperature of the Sun.”

Prof Matthew Bailes of the ARC Centre of Excellence for Gravitational Waves (OzGrav) – a collaboration of Australian universities participating in the LIGO Scientific Collaboration – said: “Never before have we seen where in the universe gravitational waves came from; the subsequent avalanche of science was virtually unparalleled in modern astrophysics”. The research has been published in Physical Review Letters, Nature and Astrophysical Journal Letters.

Prof Susan Scott, who is Leader of the General Relativity Theory and Data Analysis Group at ANU, said gravitational waves would unlock many secrets of the universe. “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.

“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.”

Neutron stars are the densest stars in the universe, with a radius of about 10 km. Scott said neutron star mergers were likely to be the site 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 said.

“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.”

Prof David McClelland of the ANU Research School of Physics and Engineering is leading a team that is developing new components and techniques for the LIGO detectors. “Using quantum mechanical techniques, we will make the largest optical sensors ever built even more powerful. We will then detect many more gravitational waves from cataclysmic events in space, involving black holes, neutron stars and things not yet known. All of this paints an incredibly bright future for the field.”