Astrophysics: Cosmic lens reveals spinning black hole.

The power of a cosmic lens to magnify and split the light from a distant, mass-accreting giant black hole into four components has allowed researchers to measure the black hole's spin. See Letter p.207 This paper reports the direct measurement of the spin of a supermassive black hole at a cosmologically significant distance. New observations of the reflection-dominated spectrum of quadruply lensed quasar more than 6 billion light years away at z = 0.658, together with an analysis of archival X-ray data, show that it is rotating rapidly. Much of its radiation comes from a compact region within three or fewer gravitational radii from the black hole. The spin of a black hole provides a record of its co-evolution with its host galaxy through cosmic time, and these new data indicate that this black hole — like those observed previously at z > 2 — grew by coherent accretion rather than in a chaotic manner.