22Aug2019

Amazing Collision Between Black Hole and Possible Neutron Star

More than 30 scientists from The ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav) are part of an international team that for the first time may have witnessed the end of a neutron star as it was engulfed by a black hole.

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Artists depiction of a black hole about to swallow a neutron star. Image courtesy of Carl Knox, OzGrav ARC Centre of Excellence

On 14 August Australian and international researchers from the LIGO (US) and Virgo (Italy) gravitational wave observatories detected the tickles in the cosmos triggered by this event, which was immediately reported on a public database available to the world’s astronomers.

To get the detectors in the high-power-shape they are in now, the World relies on the instrumental research carried out by UWA at the Gingin high optical power test facility, specifically towards solving the instabilities that limit the detector performance. On the data analysis side, Professor Linqing Wen, Dr Qi Chu and their team are running one of the five online searches that provide public alerts, which has confirmed this discovery. The Zadko telescope, directed by Associate Professor David Coward and Dr Bruce Gendre, is once more searching the skies for an electromagnetic counterpart of the gravitational wave signal.

Gravitational waves are small vibrations in the spacetime continuum caused by accelerating objects. Earlier this year, the researchers were excited when they measured a possible collision between a black hole and neutron star, but the signal was weak and could have been a false alarm. This latest signal, however, was very loud.  It was observed by both LIGO detectors and the Virgo detector, with a total false alarm rate estimated to be 1 in ten septillion years. A septillion is a one with 24 zeros after it.

Scientists would have to wait for a quadrillion (1,000,000,000,000,000) times longer than the age of the Universe to get an event like this by accident from noise alone. Based on preliminary publicly available results, this event (named S190814bv) appears to be the first Neutron Star — Black Hole merger candidate. It is estimated that the collision between the two objects occurred around 900 million light-years away, and within an area about 23 square degrees across the sky. (For comparison, the moon is about half a degree across.)

The detection was considered important not just because it would be the first time a neutron star has been seen being eaten by a black hole, which to date has only been hypothesised, but it would confirm that these sorts of mergers exist, and help us understand how such systems were formed. UWA OzGrav Postdoctoral Research Associate Dr Joris van Heijningen said as with the first Binary Neutron Star merger and subsequent kilonova detection in which Australian telescopes contributed significantly, all three gravitational wave detectors – LIGO Hanford and Livingston in the US and Virgo in Europe – were online.

“This has allowed us to pinpoint very precisely where the event occurred on the night’s sky. We hope that our astronomy friends can find a complementary signal where we have suggested they look,” Dr van Heijningen said. Australian researchers from the ARC Centre of Excellence OzGrav, which is part of the international LIGO-Virgo Collaboration that made the discovery, are now busily scouring the sky with a network of telescopes to see whether this collision also produced visible light.

They know very precisely where in the sky this event occurred. Over the next few days, they will stare at this patch of sky, and will either observe a cosmic fireball associated with a neutron star being ripped apart by a black hole, or they will see nothing. If telescopes see nothing, it might be that the neutron star was swallowed whole by the black hole without being ripped apart.

There is also the intriguing possibility that the swallowed object was not in fact a neutron star, but was a very light black hole, lighter than any other black hole we know about in the Universe. That would be a truly awesome consolation prize for the scientists.

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