Star in closest orbit ever seen around black hole

    White dwarf found orbiting around black hole in closest range ever seen, located 14,800 light years away from Earth.

    By Katie Willis on March 13, 2017

    Astronomers have found evidence of a star that orbits around a black hole every 28 minutes. This could be the tightest orbital dance ever seen by a black hole and a companion star.

    The close-in stellar couple, or binary, is located in the globular cluster 47 Tucanae, a dense cluster of stars in our galaxy about 14,800 light years away 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.

    Arash Bahramian, former PhD student, and Craig Heinke, associate professor in the Department of Physics, led the international team investigating the phenomenon. The discovery was made using NASA’s Chandra X-ray Observatory as well as NASA’s NuSTAR and the Australia Telescope Compact Array.

    “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 Bahramian, lead author on the study. “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 Heinke. “Or, the white dwarf may also completely evaporate one day.”

    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 that is orbiting a black hole at only about 2.5 times the separation between the Earth and the Moon.

    Gregory Sivakoff, assistant professor and co-author, notes, “We’ve also considered an alternative explanation, where a rapidly rotating neutron star, instead of a black hole, is tearing material from a nearby white dwarf. However, since known examples of such systems have different properties from X9, we think X9 is much more likely to contain a black hole. We’ll be keeping our eyes on X9 and other binary stars like it to test these alternative hypotheses."

    The paper, “The ultracompact nature of the black hole candidate X-ray binary 47 Tuc X9” was published in Monthly Notices of the Royal Astronomical Society in 2017.

    NASA's Marshall Space Flight Center in Huntsville, Alabama, manages the Chandra program for NASA's Science Mission Directorate in Washington. The Smithsonian Astrophysical Observatory in Cambridge, Massachusetts, controls Chandra's science and flight operations.