Black holes can rotate faster than light

How fast do supermassive black holes rotate?

Two million solar masses in rapid rotation - this is how the supermassive black hole in the center of the sixty million light-years distant spiral galaxy NGC 1365 can be described. This is the conclusion reached by an international team of researchers with the new Nustar space telescope. According to X-ray observations, the black hole rotates at more than 84 percent of the maximum possible rotational speed. The growth of the supermassive black hole must therefore have been relatively rapid, according to the scientists in the journal "Nature".

According to current knowledge, almost every galaxy contains a black hole in its center with a mass millions or even billions of times the mass of the sun. So far, astrophysicists do not know how these objects came about and how they grew to their present size. The rotation of the black holes can help unlock this secret. Because the matter falling into a black hole takes its angular momentum with it. If a black hole arises and grows relatively quickly as a result of a few events in which matter is added to it, then its rotation should be fast. However, if smaller amounts of matter flowed towards it from different directions over a long period of time, then the angular momentum would have nearly neutralized each other. The result: the black hole would rotate slowly today.

Evidence of a rapid rotation of the central black hole of the galaxy NGC 1365 had already been made on the basis of measurements with the X-ray satellites Chandra and XMM-Newton. However, the measurements from these telescopes were not accurate enough to distinguish between different possible explanations for the observed energy distribution of the X-rays. The observations of Guido Risaliti from the Harvard-Smithsonian Center for Astrophysics in Cambridge, USA, and his colleagues with the X-ray satellite Nustar, which was launched in June 2012 (Nuclear Spectroscopic Telescope Array) are now for the first time accurate enough to clearly demonstrate an extremely fast rotation of the supermassive black hole.

At least for this galaxy, the analysis provides robust evidence of rapid rotation - and thus, according to Risaliti and his colleagues, also a decisive indication of the formation and development of the black hole of NGC 1365. Observations of other galaxies must now show whether this finding can be generalized and all supermassive black holes rotate hard at the theoretical limit.