Collision forms biggest black-hole known

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Artist’s impression of two black holes merging. Photo: SXS

THE biggest black-hole known, about 80 times larger than the Sun, has been detected by an international team of scientists. 

Part of this team is the Australian National University (ANU), which plays a lead role in Australia’s involvement with the gravitational wave discovery (how the black-hole was found) through a partnership in the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO), in the United States.

Leader of the General Relativity Theory and Data Analysis Group at ANU, Prof Susan Scott, said the team discovered the four collisions by re-analysing data from Advanced LIGO’s first two observing runs.

Scientists detected the event that formed the biggest known black hole from a merger of a binary system of two black holes on July 29, 2017. The event occurred about nine billion light years away.

“This event also had black holes spinning the fastest of all mergers observed so far,” Prof Scott said.

“It is also by far the most distant merger observed.”

The three other black-hole collisions were detected between August 9 and 23 in 2017 and were between three and six billion light years away and ranged in size for the resulting black holes from 56 to 66 times larger than the Sun.

“These were from four different binary black hole systems smashing together and radiating strong gravitational waves out into space,” Prof Scott said.

“These detections of black-hole collisions greatly improve our understanding of how many binary black hole systems there are in the Universe, as well as the range of their masses and how fast the black holes spin during a merger.”

Prof Scott said after the initial observing runs were concluded, scientists recalibrated and cleaned the collected data.

“This increased the sensitivity of the detector network allowing our searches to detect more sources,” she said.

“We have also incorporated improved models of the expected signals in our searches.”

Since the second observing run finished in August 2017, scientists have been upgrading the LIGO and Virgo gravitational-wave detectors to make them more sensitive.

“This means during the upcoming third observing run, starting early next year, we will be able to detect events further out in space, meaning more detections and potentially gravitational waves from new and yet unknown sources in the Universe,” she said.

The international research team has detected gravitational waves from 10 different black-hole mergers and one neutron star collision during the past three years. Neutron stars are the densest stars in the Universe, with a diameter of up to about 20 kilometres.

Professor Scott’s research group is also designing a new project to enable them to detect gravitational waves coming from a short-lived neutron star resulting from a neutron star merger.

The results of the discoveries will be published in “Physical Review X”.

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