14 August 2019
Huge dancing jets from a spinning black hole
Published online 29 April 2019
The violent jets of matter emitted from a black hole can rapidly change direction in a matter of hours.
Binary systems containing a black hole and star often emit bright bursts of X-rays as the matter from the star is pulled in by the black hole’s gravity. What’s more, the large flows of accreting matter in disks around a black hole can produce powerful jets of ionized matter that shoot out along the hole’s axis of rotation, often moving close to the speed of light.
One such binary system, 7,800 light years from Earth, is V404 Cygni in the Cygnus constellation. It comprises a black hole about nine times the mass of our Sun and a star slightly smaller than our Sun. James Miller-Jones at the International Center for Radio Astronomy Research, Curtin University in Perth, Australia, David Russell at New York University Abu Dhabi, United Arab Emirates, and colleagues, analysed four hours of radio telescope data from 22 June 2015, when V404 Cygni was the brightest X-ray object in the sky. They calculated the angles and speeds of several mass ejections, and found that the jets showed frequent changes in orientation with a period of less than 2.6 hours.
Through detailed calculations, the researchers showed that their results could be explained by the presence of a puffed-up donut-like inner accretion disk that undergoes a relativistic form of precession: a wobble on its axis as it spins. This sort of precession could arise from a misalignment of the black hole’s spin axis with the orbital plane of the binary system, which was itself likely caused by a kick from a supernova when the black hole formed.
“The spinning black hole causes the misaligned inner accretion disk to precess, which then redirects the jets,” explains Miller-Jones. “The physics could be relatively commonplace. A good example might be a tidal disruption event, when a supermassive black hole pulls apart a star and feeds on the resulting debris.”
Miller-Jones, J. et al. A rapidly changing jet orientation in the stellar-mass black-hole system V404 Cygni. Nature http://dx.doi.org/10.1038/s41586-019-1152-0 (2019).