White dwarf star locked in tightest known orbit around black hole

White dwarf, black hole, star

Astronomers have discovered a white dwarf and a black hole locked in one of the most amazing orbital dance with the star whipping around the black hole in one of the tightest known orbits and orbiting at astonishing speed.

Researchers at University of Alberta and Michigan State University found evidence of a white dwarf star orbiting what seems to be a black hole at a distance of just 961,000 km – about 2.5 times the distance between the Earth and the Moon. This is the tightest known orbit for a black hole – white dwarf binary and according to calculations the star is completing two full orbits in an hour.

Authors of the study published in the journal Monthly Notices of the Royal Astronomical Society believe that instead of the white dwarf going into oblivion, the star will remain in orbit. However, its fate can’t be ascertained because of its close proximity to the black hole.

The stellar system, known as X9, is located in the globular cluster 47 Tucanae, a dense cluster of stars in our galaxy about 14,800 light years away from the Earth. This discovery was made using NASA’s Chandra X-ray Observatory as well as NASA’s NuSTAR and CSIRO’s Australia Telescope Compact Array (ATCA).

The system has been under observation for quite a few years now, but it was for the first time that radio observations with the ATCA in 2015 revealed that the pair likely contains a black hole pulling material from a companion white dwarf.

NASA revealed that there are changes in the X-ray brightness of the X9 in the same manner every 28 minutes, which is likely the length of time it takes the companion star to make one complete orbit around the black hole. Chandra data also shows evidence for large amounts of oxygen in the system, a characteristic feature of white dwarfs. A strong case can, therefore, be made 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.

One of the obvious questions would be how did a white dwarf end up being so close to the black hole? One of the possible answers could be 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, NASA says.

An alternative explanation for the observations is that the white dwarf is partnered with a neutron star, rather than a black hole.