Scientists spot black hole spinning at half the speed of light

Astronomers have discovered a distant supermassive black hole that spins fast. Really fast.

It sits at the center of a quasar, an extremely bright galactic core that surrounds a 'supermassive' black hole some 6 billion light years away. The spin rate of this quasar, called RX J1131-1231, is "about 90 percent of the maximum value allowed by General Relativity that approximates half the speed of light," Mark Reynolds from the University of Michigan, who was part of the team that carried out the research, told the Monitor. 

The blazingly fast spin rate indicates that this black hole has grown "via mergers, rather than pulling material in from different directions," according to a press release from NASA. Bombardment with materials coming from different directions would have reduced its spin rate significantly

As the black holes grow in size, they form a swirling disk of gas and dust around them. "The inner regions of the disk grow very hot and a corona of hot gas and electrons form above the disk," at temperatures of millions of degrees Dr. Reynolds says.

This corona acts as the source of ionizing X-ray radiation that bounces off the inner disk. These are the X-rays that the scientists measured to determine the spin rate of the black hole, according to a paper published in the journal Nature by Reynolds and his colleagues titled "Reflection from the strong gravity regime in a lensed quasar at redshift z = 0.658." 

"We estimate that the X-rays are coming from a region in the disk located only about three times the radius of the event horizon, the point of no return for infalling matter," said Jon M. Miller from the University of Michigan, another author on the paper. "The black hole must be spinning extremely rapidly to allow a disk to survive at such a small radius."

Under normal circumstances, the X-rays would be too faint to measure. But a giant elliptical galaxy along the line of sight to the quasar acted as a 'gravitational lens,' that magnified the radiation. Often used as a proof to support Einstein's General Theory of Relativity, which explains gravity by describing how massive objects cause space and time to "curve," gravitational lensing results when a massive galaxy comes between the source of light and the observer. 

"Because of this gravitational lens, we were able to get very detailed information on the X-ray spectrum – that is, the amount of X-rays seen at different energies – from RX J1131," said co-author Mark Reynolds also of Michigan. "This in turn allowed us to get a very accurate value for how fast the black hole is spinning."

The ability to measure black hole spin will help scientists understand how black holes grow and if they evolve at about the same rate as their host galaxies, Reynolds says.