In a first of its kind discovery, astronomers have spotted a white dwarf which they say is circling a black hole in so tight an orbit that it completes two full orbits around the black hole in an hour.
The discovery was made by scientists at University of Alberta and Michigan State University who reveal in their study published in journal Monthly Notices of the Royal Astronomical Society that they have found evidence of a white dwarf locked in an orbit around a black hole that is the tightest orbit till date. Researchers say that the white dwarf is a distance of 961,000 km – about 2.5 times the distance between the Earth and the Moon – and in cosmic terms this distance is pretty small.
Despite the white dwarf being so close to the black hole, it will not be pulled by the black hole into oblivion astronomers believe and instead will remain in orbit. However, there is all the possibility that the star could eventually be starved off all its material and could eventually evaporate completely.
The white dwarf is located in a stellar system, known as X9, 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.