The star is in the closest orbit ever seen around a black hole.
Credit: X-ray: NASA/CXC/University of Alberta/A.Bahramian et al.; Illustration: NASA/CXC/M.Weiss
Astronomers have found evidence of a star that whips around a black hole about twice an hour. This could be the tightest orbital dance ever seen by a black hole and a companion star.
This discovery was made using NASA's Chandra X-ray Observatory as well as NASA's NuSTAR and the Australia Telescope Compact Array.
Tom Maccarone, an associate professor in the Texas Tech University Department of Physics and Astronomy, came up with the approach that gives scientists the strongest argument that this source is an accreting black hole. He also was involved in the proposal writing, determining the data analysis strategy, interpreting the data and writing the final results study.
"Almost exactly a decade ago, I led the group that found the first strong candidate black hole in a globular cluster," Maccarone said. "That object was in a distant galaxy where we couldn't study it in as much detail as this source, but was a much brighter X-ray source. What is exciting about the connection between that discovery and this discovery is that the two sources both appear to have white dwarf stars supplying the gas the black holes are swallowing, and what's more is that the white dwarfs appear to be rich in carbon and oxygen, rather than helium, so they must have been massive white dwarfs when they formed."
The close-in stellar couple – known as a binary – is located in the globular cluster 47 Tucanae, a dense cluster of stars in our galaxy about 14,800 light years from Earth.
While astronomers have known about this binary for many years, it wasn't until 2015 that radio observations revealed the pair likely contains a black hole pulling material from a companion star.
New Chandra data of this system, known as X9, show that it changes in X-ray brightness 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. This, plus Chandra data that shows evidence for large amounts of oxygen in the system, makes a strong case that X9 contains a white dwarf star orbiting a black hole at only about 2.5 times the separation between the Earth and the Moon.
"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," said the study's first author, Arash Bahramian of the University of Alberta in Edmonton, Canada. "Luckily for this star, we don't think it will follow this path into oblivion, but instead will stay in orbit."
Although the white dwarf does not appear to be in danger of falling in or being torn apart by the black hole, its fate is uncertain.
"Eventually so much matter may be pulled away from the white dwarf that it ends up becoming an exotic kind of planet," said co-author Craig Heinke, also of the University of Alberta. "Or, the white dwarf may also completely evaporate one day."
How did the black hole get such a close companion? One possibility is that the black hole smashed into a red giant star, then gas from the outer regions of the star was ejected, forming a binary containing a black hole and a white dwarf. 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 by the Laser Interferometer Gravitational-Wave Observatory. It could potentially be detected with future gravitational wave observatories in space.
An alternative explanation for the observations is that the binary contains a neutron star spinning faster as it pulls material from a companion star via a disk. This process can decrease the rotational period of the neutron star to a few milliseconds. A few such objects, called transitional millisecond pulsars, have been detected. 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.
"We're going to watch this binary closely in the future, since we know little about how such an extreme system should behave," said co-author Vlad Tudor of Curtin University in Perth, Australia. "We're also going to keep studying globular clusters in our galaxy to see if more evidence for very tight black hole binaries can be found."
A paper describing these results was recently accepted for publication in the Monthly Notices of the Royal Astronomical Society and is available online. NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington, D.C. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.
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