New research uncovers secrets of space’s mysterious black holes

X-ray: NASA/CXO/CSIC-INTA/G.Miniutti et al.; Illustration: NASA/CXC/M. Weiss;

Scientists have uncovered clues into the reasons why mysterious black holes grow in size, after discovering an amazing struggle between a star and a black hole 250,000 light years away.

Professor Andrew King, Professor of Theoretical Astrophysics in the University of Leicester’s School of Physics and Astronomy discovered a white dwarf star that escaped being swallowed by a black hole, but instead became trapped in orbit around it.

Grazing encounters between stars and black holes like this should be more common than direct collisions, but very few have been recorded. This could be because supermassive black holes simply swallow passing stars and grow in size, or surviving stars often take too long to orbit a black hole for astronomers to recognise that the x-ray eruptions repeat.

Professor Andrew King, Professor of Theoretical Astrophysics in the University’s School of Physics and Astronomy, said: “This white dwarf is locked into an elliptical orbit close to the black hole, orbiting every nine hours. At its closest approach, about 15 times the radius of the black hole’s event horizon, gas is pulled off the star into an accretion disk around the black hole releasing X-rays which the two spacecraft are detecting.

“In astronomical terms, this event is only visible to our current telescopes for a short time – about 2,000 years, so unless we were extraordinarily lucky to have caught this one, there may be many more that we are missing elsewhere in the Universe.

“Such encounters could be one of the main ways for black holes the size of this one to grow.”

The black hole studied is small and the white dwarf’s orbit is sufficiently close that the x-ray eruptions are identifiable.

Originally a red giant, the hydrogen-rich outer layers of the star were stripped away by the black hole, leaving its helium-rich core – a white dwarf.

Professor King continued: “In principle, this loss of mass would continue until and even after the white dwarf became a planet, in about a trillion years. This would be a remarkably slow and convoluted way for the Universe to make a planet!”

“It will try hard to get away, but there is no escape. The black hole will eat it more and more slowly, but never stop.”

Data from two orbiting X-ray telescopes, NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton, to study a black hole at the centre of galaxy GSN 069, was analysed.

Launched in 1999, XMM-Newton carries the European Photon Imaging Camera (EPIC) which includes components built at the University of Leicester.

Professor King’s analysis is published in Monthly Notices of the Royal Astronomical Society.