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Closest-yet star to black hole

Astronomers have found a star that whips around a black hole about twice an hour. That’s a very close orbit, possibly the tightest orbital dance ever witnessed for a star and black hole.

Scientists said on March 13, 2017 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. The system is located some 14,800 light-years from Earth in 47 Tucanae, one of the dense symmetrical globular clusters associated with our Milky Way galaxy. The black-hole-and-star system is known as X9. New data from the Chandra X-ray Observatory show that it changes in X-ray brightness every 28 minutes, making it likely the companion star makes one complete orbit around the black hole in this amount of time. A statement from Michigan State University (MSU) explained:

Chandra data also shows evidence for large amounts of oxygen in the system, a characteristic of white dwarfs. A strong case can, therefore, be made that that the companion star is a white dwarf, which would then be orbiting the black hole at only about 2.5 times the separation between the Earth and the moon.

They made the discovery using NASA’s Chandra X-ray Observatory as well as NASA’s NuSTAR and the Australia Telescope Compact Array.

Arash Bahramian, lead author with the University of Alberta (Canada) and MSU, said:

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.

Luckily for this star, we don’t think it will follow this path into oblivion, but instead will stay in orbit.

These scientists’ paper has been accepted by the peer-reviewed journal Monthly Notices of the Royal Astronomical Society.

X9 – the close star-and-black hole pair – lie 14,800 light-years away in the globular star cluster 47 Tucanae, shown here. Image via Fred Espenak at AstroPixels.com. Used with permission.

How did the black hole get such a close companion? The MSU statement explained:

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. The remaining core of the red giant would form into a white dwarf, which becomes a binary companion to the black hole. The orbit of the binary would then have shrunk as gravitational waves were emitted, until the black hole started pulling material from the white dwarf.

The gravitational waves currently 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. Sources like X9 could potentially be detected with future gravitational wave observatories in space.

An alternative explanation for the observations is that the white dwarf is partnered with a neutron star, rather than a black hole. 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. A few such objects, called transitional millisecond pulsars, have been observed near the end of this spinning-up phase. The authors do not favor this possibility as transitional millisecond pulsars have properties not seen in X9, such as extreme variability at X-ray and radio wavelengths. However, they cannot disprove this explanation.

Artist’s concept of a star (the white object on the far left) exceedingly close to a black hole. Material from the star is being ripped from it by the black hole’s powerful gravity, and this material is feeding a disk swirling around the hole. Where the material strikes the disk, there’s a bright spot. Image via MSU.

Bottom line: The star black hole pair X9 – in the globular star cluster 47 Tucanae – is now the closest known such pair. The star is thought to whip around the black hole every 28 minutes.


Deborah Byrd