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Credit: J. Bergeron, Sky & Telescope Magazine

GRO J1655-40 and GRS 1915+105 are superluminal jet sources ("microquasars") with compact objects that are postulated to be black holes spinning near the extreme theoretical limit. Frame dragging, a prediction of General Relativity, should cause the accretion disk to precess---an observable effect in RXTE data, according to the predictions of Wei Cui and colleagues. For GRO J1655-40, with both the black hole spin and mass (about 7 solar masses) known, the theoretically expected precession frequency of the inner portion of the accretion disk was calculated due to frame dragging. The result is ~300 Hz, which is precisely the observed frequency at which X-ray brightness varies nearly periodically.