Black hole's spin is at the limits of possibility

Posted by Ken Kola

NASA's Nuclear Spectroscopic Telescope Array (NuSTAR) and the European Space Agency's XMM-Newton, have together accurately measured, for the first time, the spin rate of a black hole with a mass two million times that of our sun.

The supermassive black hole lies at the heart of a galaxy called NGC 1365, and is spinning almost as fast as Einstein's theory of gravity will allow. The findings will lead to a better understanding of how black holes and galaxies evolve, and are also are a powerful test of Einstein's theory of general relativity.

"This is hugely important to the field of black hole science," said Lou Kaluzienski, a NuSTAR program scientist at NASA Headquarters in Washington.

NuSTAR, which launched in June 2012, is designed to detect the highest-energy X-ray light in great detail. It complements telescopes that observe lower-energy X-ray light, such as XMM-Newton and NASA's Chandra X-ray Observatory. Scientists use these and other telescopes to estimate the rates at which black holes spin.

However, until now, these measurements were uncertain because clouds of gas could have been obscuring the black holes and confusing the results. But with help from XMM-Newton, NuSTAR was able to see a broader range of X-ray energies and penetrate deeper into the region around the black hole.

The new data demonstrate that X-rays aren't being warped by the clouds, but by the tremendous gravity of the black hole. It also shows that spin rates of supermassive black holes can be determined conclusively.

Measuring the spin of a supermassive black hole is fundamental to understanding its past history and that of its host galaxy.

"These monsters, with masses from millions to billions of times that of the sun, are formed as small seeds in the early universe and grow by swallowing stars and gas in their host galaxies, merging with other giant black holes when galaxies collide, or both," says Guido Risaliti of the Harvard-Smithsonian Center for Astrophysics.

Supermassive black holes are surrounded by pancake-like accretion disks, formed as their gravity pulls matter inward. Einstein's theory predicts that the faster a black hole spins, the closer its accretion disk will be. The closer the accretion disk is, the more gravity from the black hole will warp the X-ray light from the disk - and NuSTAR's higher-energy X-ray data showed that the iron in the accretion disk was so close to the black hole that its gravity must be causing the warping effects.

Now that obscuring clouds have been ruled out, scientists can now use the distortions in the iron signature to measure the black hole's spin rate.