Correlated Radio-X-Ray Variability of Galactic Black Holes: A Radio-X-Ray Flare in Cygnus X-1

We report on the first detection of a quasi-simultaneous radio-X-ray flare of Cygnus X-1. The detection was made on 2005 April 16 with pointed observations by the RXTE and the Ryle telescope, during a phase where the black hole candidate was close to a transition from its soft state to its hard state. The radio flare lagged the X-rays by ~7 minutes, peaking at 3:20 hr barycentric time (TDB 2,453,476.63864). We discuss this lag in the context of models explaining such flaring events as the ejection of electron bubbles emitting synchrotron radiation.