Bright lights in triangular pattern 'hang' in sky, then accelerate and fade from view.I was flying as co-pilot in a Cessna Citation II (am myself an ATP-rated Learjet captain) westward at 39000 feet when I saw three bright lights in a triangular pattern (not equilateral -- more like a right triangle, though that may be a matter of orientation of the object) simply hanging in the sky to the north near Cassiopeia, at an estimated altitude of 45-49000 feet. I asked my partner if he was watching it and he said yes. The lights were all of a greater relative magnitude than Sirius -- perhaps equalling or greater than Venus in brightness. After about 5 to 10 seconds, the lights began to move eastward, accelerating rapidly and fading from view simultaneously (may have been receding?). No aircraft in my experience has ever accelerated so quickly. We believe it was one object -- though no details could be made out between the lights -- else the three separate objects would have had to have been maintaining a precise formation during acceleration. Each of the observers have been flying aircraft for over 15 years, and neither of us had ever seen anything like this.