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 .)...
Philip F. Schewe; Evidence of black hole rotation. Physics Today 1 July 2001; 54 (7): 9. https://doi.org/10.1063/1.4796431
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