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Black hole swallowing a neutron star is likely source of gravitational wave 'chirp' detected last we

Aug 26, 2019

The scientists detected signals from two colliding objects about five and three times the mass of our Sun.

 

(Supplied: Carl Knox/OzGrav ARC Centre of Excellence)

Tiny ripples detected in the cosmos last week were likely to have been created in the final moments before a black hole gobbled up a small dense star, scientists say.

Key points

Scientists believe they may have detected gravitational waves caused by a black hole gobbling up a dead star

If confirmed this would complete a trifecta of major gravitational wave discoveries

If it isn't a neutron star, it could be something unknown to science

If confirmed, it would be the first time gravitational waves from such an event have been detected.

On August 14 the international team of scientists detected the telltale chirp as two massive objects collided about 900 million light years away.

The signal was detected by both the Advanced Laser Interferometer Gravitational-wave Observatory (LIGO) in the US and the Virgo detector in Italy.

 

Latest @LIGO @ego_virgo update on #S190814bv labels it "NSBH". This means that, if confirmed, our candidate event *could* be an exciting new milestone: the first ever detection of a #NeutronStar #BlackHole merger. But, as always in science, we must be cautious... 1/5

"It was a very clear and loud signal," said astrophysicist Susan Scott of the Australian National University and OzGrav's chief investigator.

"We have to do some more checking of the data and signal to be absolutely certain but it is looking quite probable."

Professor Scott said the mass of the objects indicated that one of the objects had to be a black hole and the other a neutron star — the dead core of an exploded star.

"We've never seen a black hole less than five solar masses," she said.

"The lighter one should be a neutron star. They are less than three solar masses in general."

If they are right, the latest detection would complete a trifecta for the scientists.

Gravitational waves are created in the moments before two objects of a large mass collide.

The three gravitational wave detectors sense miniscule movements in space-time using laser beams travelling through perpendicular pipes.

In 2015 the LIGO team detected gravitational waves created by the merging of two black holes. Two years later the LIGO and Virgo teams detected the collision of neutron stars for the first time.

Earlier this year, the LIGO team thought they may have detected a black hole eating a neutron star but this was later put down to terrestrial noise, said Professor Scott.

So how do we know if it's a black hole and neutron star?

While the signal is stronger this time around, and pinpointed to a relatively small patch of sky, follow up observations by telescopes including ANU's Skymapper telescope have found no evidence of a star being shredded.

If two objects around the same mass spiral in towards each other there should be some fireworks.

"We've looked for light signatures of the event but no-one has found any up to this point," Professor Scott said.

"That indicates that if it is a black hole and a neutron star then very likely the neutron star has been swallowed whole by the black hole."

This could happen, she said, if the objects were different masses.

"The smaller object gets sucked in more quickly and so it is swallowed whole."

Professor Scott said it would take another few months to be entirely certain of the results.

It could even be possible, she said, that they had witnessed something never seen before, such as a very light black hole.

"If that were the case, that would open up a whole new area because how would black holes of that mass actually form?" she said.

"That would be a very exciting consolation prize if it's not actually a neutron star."

 

Source:https://www.abc.net.au/news/science/2019-08-19/gravitational-wave-signs-of-neutron-star-black-hole-merger/11427076

 

 

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