If you need another reason not to trust black holes, this is it. Astronomers have discovered an unusual binary system consisting of a black hole being circled by a tiny white dwarf star and it appears the black hole is keeping the small dying star in a tight orbit and slowly siphoning off its energy rather than just gulping it down and putting an end to its misery.
The binary system is X9, located in the dense globular star cluster 47 Tucanae about 14,800 light years away from Earth in the Milky Way galaxy. The system provided a number of surprises to the astronomers from Michigan State University and the University of Alberta (Canada) who discovered it. “Correctly identified it” might be a better description, since the existence of the binary system had been known but it wasn’t until 2015 that the pair were determined to be a black hole and a white dwarf. Using NASA’s Chandra X-ray Observatory, NASA’s NuSTAR and the Australia Telescope Compact Array, they saw a large amount of oxygen in the small star, which identified it as a white dwarf. It was also a surprise to find a black hole in a globular star cluster.
The most interesting discovery was that the white dwarf appears to be in the tightest orbit ever seen in such a system. According to their paper recently accepted by the Monthly Notices of the Royal Astronomical Society, the astronomers noticed a change in the system’s X-ray brightness every 28 minutes, which would be caused by a 28 minute orbit. At that rate, the white dwarf is spinning around the black hole at a distance of 2.5 times the separation between the Earth and the moon.
At that distance, the black hole appears to be toying with the white dwarf, says study lead author Arash Bahramian.
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.
It seems like this white dwarf has had more bad luck than good. One theory on the formation of X9 has a red giant star colliding with the black hole, losing its gas and nuclear fuel and becoming the white dwarf. The other theory is even worse – the dying white dwarf is in a dead star tango with a neutron star, the dense corpse of a collapsed giant star. Suddenly, a close relationship as the toy of a black hole doesn’t sound quite so bad.
The astronomers will continue to watch X9 and wager on the eventual fate of the white dwarf. Meanwhile, the Dense Globular Star Cluster would be a great name for a band.