White Dwarf Star In Closest Orbit Ever Seen With Black Hole
In the binary system known as 47 Tucanae X9, located about 14,800 light years from Earth, a white dwarf star speeds around a black hole about twice an hour, according to NASA.
A team of scientists made this discovery using NASA’s Chandra X-ray Observatory together with NASA’s Nuclear Spectroscopic Telescope Array (NuSTAR) and CSIRO’s Australia Telescope Compact Array (ATCA). The Royal Astronomical Society will publish the research in an upcoming issue of Monthly Notices.
Chandra’s latest data of this binary system shows that its X-ray brightness changes every 28 minutes. This likely indicates the length of time the companion star completes an orbit around the black hole. Since there is evidence of the system containing large amounts of oxygen, the companion star could be a white dwarf.
With the star so close to the black hole, the latter’s gravitation pulls material away from the star, said Arash Bahramian of the University of Alberta. The matter then joins a disk of matter before falling into the black hole, Bahramian added. “Luckily for this star, we don’t think it will follow this path into oblivion, but instead will stay in orbit.”
White Dwarf’s Fate Uncertain
However, the white dwarf’s fate remains uncertain. Co-author Craig Heinke, also from the University of Alberta, said the black hole could pull away so much matter from the star. It could end up with having the mass of a planet or even worse.
“If it keeps losing mass, the white dwarf may completely evaporate,” Heinke added.
The paper’s authors believe the binary system formed after the black hole crashed into a red giant star. After the collision, the star expelled the gas from its outer regions. Then, the remaining core formed into a white dwarf and became the black hole’s companion.
On the other hand, scientists presented another explanation for the binary system. They said the white dwarf might have partnered with a neutron star, not a black hole. A neutron star results from the gravitational collapse of a massive star. A neutron stars attributes include strong gravity, powerful magnetic and electric fields, and high velocities.
Under this scenario, they said, the neutron star spins faster, pulling materials from a companion star via a disk. This process could “lead the neutron star spinning around its axis thousands of times every second.” Scientists have observed similar objects, called transitional millisecond pulsars. These pulsars have extremely variable X-ray and radio wavelengths.
The authors do not favour this possibility since the X9 binary system does not share these properties. However, they could not disprove this scenario.