Star spotted in closest orbit ever around black hole

Janie Parker
March 17, 2017

The discovery of this star - part of a binary system including a white dwarf - suggests what could well be the tightest orbital dance ever witnessed for a likely black hole and a companion star, according to researchers.

Evidence has been discovered of a star orbiting a black hole at just 2½ times the distance between the Earth and moon.

"Luckily for this star, we don't think it will follow this path into oblivion - it should stay in orbit".

Hence, the star black hole pair X9 - in the globular star cluster 47 Tucanae - is now the closest known such pair.

"For a long time, it was thought that X9 is made up of a white dwarf pulling matter from a low mass Sun-like star", said researcher Arash Bahramian.

The black hole and its star companion were located in the globular cluster 47 Tucanae-a cluster of stars 14,800 light years away from Earth.

"If it keeps losing mass, the white dwarf may completely evaporate". These objects tend to pack roughly the mass of the sun into a body only slightly larger than Earth, making them about 200,000 times denser than our planet.

And what if it's not actually a black hole?

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Astronomers made the discovery using the Australia Telescope Compact Array, operated by CSIRO near Narrabri in NSW, backed up with data from two of NASA's space telescopes, Chandra X-ray Observatory and NuStar. "This theory doesn't explain everything we're seeing here, so our best current explanation is that we're dealing with a white dwarf in extremely close proximity to a black hole".

The researchers think this system would be a good candidate for future gravitational wave observatories to observe.

Another possible explanation for the existence of this unusual stellar pair is that the white dwarf is actually orbiting a neutron star, not a black hole. Over time, the emission of gravitational waves has brought the two objects into their tight orbital dance.

Using the telescoped on the ground, the astronomers were able to perceive the fluctuations in X-ray in the white dwarf/black hole binary system, which is called the X9.

Black holes rip stars apart.

Black holes are so freakish, they sound unreal.

Black Hole Quiz: How Well Do You Know Nature's Weirdest Creations? In this scenario, the neutron star spins faster as it pulls material from a companion star via a disk, a process that can decrease the rotational period of the neutron star to a few thousandths of a second. Such objects, called transitional millisecond pulsars, have been observed but some of their properties don't match up with X9. 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. But they can't rule out this hypothesis yet.

Even though astronomers have known about the binary for several years, it was only in 2015 that the team of scientists from Curtin University and the International Centre for Radio Astronomy Research (ICRAR), found that it was possibly composed of materials being pulled by the black hole from another companion star. He's the lead author of a recent paper in Monthly Notices of the Royal Astronomical Society detailing the team's findings.

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