Closest Star Around A Black Hole Discovered

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Scientists said on March 13 that they've found the closest yet star to a black hole, with the tightest orbital dance ever seen for a star-and-black-hole pair.

Astronomers found a star circling a massive black hole, just about 2.5 times the distance between the Earth and the Moon.

"This white dwarf is so close to the black hole that material is being pulled away from the star and dumped onto a disk of matter around the black hole before falling in", said Arash Bahramian, lead author with the University of Alberta and MSU, in a press release.

Slavko Bodganov, an associate research scientist and co-author of the paper, said that the combination of the unrivaled capabilities of these telescopes led to the discovery of the unusual cosmic pair that sits in a star cluster about 14,800 light-years away.

The binary system of the star and black hole is called X9, and the black hole within the system appears to be a stellar-mass black hole, which is relatively small.

While the existence of the binary known as 47 Tuc X9 was already known, the discovery of the new evidence was made using deep space telescopes belonging to Nasa and The Commonwealth Scientific and Industrial Research Organisation (CSIRO).

The duo are in the globular cluster 47 Tucanae, a dense cluster of stars in our galaxy about 14,800 light years away from Earth. The star's remaining core then became a white dwarf and settled into orbit around the black hole.

"Eventually so much matter may be pulled away from the white dwarf that it ends up becoming an exotic kind of planet", said Craig Heinke, an associate professor in the Department of Physics who helped lead the global team investigating the phenomenon. A high level of oxygen in X9 suggests that the star is a white dwarf.

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No star before has been discovered lingering so near a black hole.

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.

"In 2015, Dr. Miller-Jones and collaborators observed strong radio emission from X9 indicating the presence of a black hole in this binary", Bahramian continued.

The star is also travelling at 6.3 million kilometres per hour, or one per cent the speed of light, meaning it takes just 30 minutes to complete an orbit around the black hole. Yet astronomers have found good evidence they exist. Astronomers have known about the system called X9 for many years but it was not until now that they were able to observe the subtle changes in its orbit.

The new study has been accepted for publication by the journal Monthly Notices of the Royal Astronomical Society.

"One possibility is that the black hole smashed into a red giant star, and then gas from the outer regions of the star was ejected from the binary".

Such gravitational waves are too low-frequency to be detected by the Laser Interferometer Gravitational-Wave Observatory (LIGO), which recently spotted waves from several different black-hole mergers, study team members said. 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.

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