New method to measure black hole spin rate raises questions

Nature : Black holes have two fundamental characteristics—mass and spin. Mass is relatively easy to measure. Spin, a black hole’s angular momentum, is much more difficult. The current technique involves evaluating the iron emission line present in x rays emitted by clouds of hot ionized gas that surround the black hole. Some of those x rays get reflected off the black hole’s accretion disk and travel toward Earth, where they can be measured. The wider the emission line, the higher the black hole’s spin. That higher spin allows the accretion disk to be closer to the event horizon, and thus the gravity of the black hole is more able to distort the emission spectrum. A new technique developed by Christine Done of Durham University, UK, and her colleagues measures lower-energy x rays emitted directly from the accretion disk. The spectra of those x rays provide the disk’s temperature measurements, which can be correlated to the distance of the accretion disk from the event horizon. The black hole they measured had a spin of at most 86% the relativistic upper limit for black hole rotation. The older method, which has not been used on their target, has only produced measurements of greater than 90%. The difference between the results suggests two possibilities: Either one of the methods is more accurate than the other, or a greater variance in black hole spin exists than previously believed.