Research conducted using computer simulations has demonstrated how a rapidly-spinning black hole not only engulfs matter, it also emits energy in the form of relativistic jets. Thanks to a revolutionary faster method of calculation, astronomers at the University of Amsterdam were able to demonstrate that it is possible for these jet streams to gradually change direction (tilt) as a result of space-time being dragged into the rotation of the black hole. The results will be published in the Monthly Notices of the Royal Astronomical Society.
Rapidly-spinning black holes ‘shoot’ matter out into space at nearly the speed of light. This is because the matter spiralling around the black hole is interlaced with magnetic fields. The black hole drags the curved space-time into its rotation and, in doing so, wraps magnetic fields around itself. This creates a kind of launching tube from which energy is emitted: the relativistic jets.
The spiralling material surrounding a black hole forms a rotating disk. This disk often rotates on a different axis than the black hole itself. Astronomers think that the dragging effect causes the disk to tilt around the axis on which the black hole turns. This is called precession. The fact that the jets tilt along with the disk may explain the fluctuations in the intensity of infrared light around black holes, the so-called quasi-periodic oscillations, or QPOs. This is similar to the way in which the rotating beam of a lighthouse appears to speeds up as it approaches an observer. QPOs were first discovered near black holes (as X-rays) in 1985 by Michiel van der Klis, who is a co-author of the present article.
The reason why the tilting of the jets was not discovered earlier is that 3D simulations of the area around a rapidly-spinning black hole require an enormous amount of computational power. The effects occur at both a small scale (magnetic turbulence in the disk) and a large scale (relativistic jets). Furthermore, all possible complications of Einstein’s theory of gravity must be taken into account in the calculations. Chief author Matthew Liska, a PhD candidate under Van der Klis, has spent the past three years developing a new simulation code that can conduct the calculations much more quickly.
With the help of the American supercomputer Blue Waters, a resolution was achieved that was higher than ever before, up to a billion pixels. This computer contains thousands of graphic cards that were originally designed for the gaming industry. 'Each of these graphic cards, in turn, contains thousands of little calculators. The challenge is to make efficient use of these and to get them to communicate with one another,' Liska explains. Second author Casper Hesp, a Master’s student in Astronomy and Neuroscience at the UvA, used the same supercomputer to convert the simulations into images that clearly show the jets changing direction.
These results are important for further calculations involving rotating black holes, which are currently being conducted all over the world. Through these efforts, astronomers are attempting to understand recently discovered phenomena such as the merging of double black holes and regular stars being engulfed by supermassive black holes.
The calculations are also being applied in interpreting the observations of the Event Horizon Telescope (EHT), which captured the first recordings of the supermassive black hole in the centre of the Milky Way. Co-author Sera Markoff, Hesp’s supervisor and member of the EHT Science Council: 'What we want to find out is whether Einstein’s predictions, which until now have always proved correct, are also entirely accurate with regard to the extreme gravity that exists near a rotating black hole. These kinds of simulations are essential in order to bridge the gap between theory and observation.'