Tightest Orbital Dance Between Black Hole And Star Witnessed By Astronomers

Posted March 17, 2017

Astronomers have just spotted a star whizzing around a vast black hole at about 2.5 times the distance between Earth and the Moon, and it takes only half an hour to complete one orbit. One possibility is that the black hole might have collided with a red giant star at some stage of its life, consuming all the gas around it and turning it into a white dwarf.

That's because it has the closest orbit ever seen between a black hole and a companion star.

"We think the star may have been losing gas to the black hole for tens of millions of years and by now has now lost the majority of its mass", said researcher James Miller-Jones from Curtin University and the International Centre for Radio Astronomy Research. Researchers subsequently chose to combine the superb capabilities of the three telescopes to study the relatively unexplored binary in unprecedented detail.

Astronomically speaking, at a million kilometres, that's very, very close.

Researchers from the Michigan State University and the University of Alberta conducted a study using the Chandra X-ray Observatory, the Australia Telescope Compact Array (ATCA) and NASA's NuStar space telescope to make the discovery.

A binary system in the globular cluster 47 Tucanae - about 14,800 light-years away from Earth but inside the Milky Way - has been known for years, but it was only in 2015 that astronomers found one of the stellar bodies was a black hole.

Data taken from a telescope array near Narrabri in NSW has convinced astronomers that the star system 14,000 light years away is most likely that of a white dwarf and black hole locked in a tight orbital dance.

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Black holes are famous for their magical power of destroying the stars due to unequal gravitational forces.

How did a star get so close to a black hole?

A white dwarf star almost 15,000 light-years from Earth apparently whips around its companion black hole once every 28 minutes, a new study suggests.

While analyzing data collected about two other, unrelated X-ray binaries, Bogdanov and co-researchers using the Chandra X-ray Observatory realized the data could be utilized to further study the peculiar X9 system. The X9 contains massive oxygen amounts per evidence, which suggests that it houses a white dwarf star. A study detailing the findings has been accepted for publication in the Monthly Notices of the Royal Astronomical Society and is available online. The orbit of the binary would then have shrunk as gravitational waves were emitted, until the black hole started pulling material in from the white dwarf.

Globular clusters have much higher stellar density than other galactic space, so a collision between a star and the black hole is the most likely process.

The gravitational waves now being produced by the binary have a frequency that is too low to be detected with Laser Interferometer Gravitational-Wave Observatory, LIGO, that has recently detected gravitational waves from merging black holes.

In such pairings, neutron stars rotate faster and faster as they pull material from their companions, sometimes spinning on their axes thousands of times per second. This possibility is less likely based on the extreme variability seen from the X-ray and radio observations; however, the researchers can not yet disprove this explanation and plan to continue studying X9 to better understand the properties of such extreme systems.