Evidence of black hole rotation

An object in our galaxy called GRO J1655–40 consists of a seven-solar-mass black hole gradually devouring a nearby normal-star companion. Matter from the star first collects in an accretion disk orbiting the black hole before taking the final plunge through the event horizon, emitting x rays along the way. General relativity predicts the innermost stable orbit for matter circling this black hole, if it were non-rotating, to be about 64 km. However, at the April American Physical Society meeting, Tod Strohmayer of NASA’s Goddard Space Flight Center reported the discovery of a 450-Hz oscillation from the black hole in archival data from the Rossi X-Ray Timing Explorer. This high frequency indicates matter is orbiting the black hole at a radius of only 49 km. Based on this finding, he concluded that the black hole itself must have angular momentum. (T. Strohmayer, Astrophys. J. 552, L49, 2001 .)