RELATIVISTIC MOTION IN A NEARBY BRIGHT X-RAY SOURCE

THE recent discovery(1) of radio components apparently moving away fro m a Galactic source of transient X-ray emission faster than the speed of light (superluminal motion) has identified a low-energy Galactic co unterpart to quasars. Here we report high-resolution radio observation s of a second Galactic superluminal radio source GRO J1655-40, which w as detected as an X-ray transient(2) on 27 July 1994. Our radio images reveal two components moving away from each other at an angular speed of 65 +/- 5 mas d(-1), corresponding to superluminal motion at the es timated distance of 3-5 kpc. The 12-day delay between the X-ray and ra dio outbursts suggests that the ejection of material at relativistic s peeds occurs during a stable phase of accretion onto a black hole, whi ch follows an unstable phase with a high accretion rate,