Two adult males witness extremely bright, moving light in the eastern sky; not Iridium satellite, or ISS.Two adult males were standing outside, looking toward the eastern sky. Conditions were unusually clear, with no cloud coverage. The stars were beautifully prominent.One of the witnesses observed a very bright light in the eastern sky, which initially he thought was a satellite. The light was at an azimuth of approximately 80-100 degrees (true); angle of elevation was approximately 45-60 degrees (above the horizontal plane).The first witness called the unusual looking light to the attention of the other witness present, and they both stood, watching the object, as it coursed generally to the north, i.e., from their right to left. It followed a flight path that was generally parallel to a horizontal line. It was moving at approximately the same (angular) velocity that is seen for a typical satellite, i.e. approximately ½-1 degree of arc per second.Suddenly, the light, which already was quite bright, e.g. brighter than the ISS when it passes directly overhead, flared up dramatically. Following this dramatic increase in luminosity, and over the course of approximately 10-15 seconds, the brilliance of the object subsided to the size and illumination of a typical prominent low-orbit satellite, and the object continued its flight to the north.It was still visible at approximately an azimuth of 20 degrees (true), and at approximately 20-30 degrees angle of elevation.I have checked www.Heavens-Above.com, and no Iridium satellite was visible from the witnesses’ location for the evening of April 20, 2009. Also, the ISS was not visible from the location on this date, and no Space Shuttle was in orbit on this date.My impression is that the object was not a tumbling booster (ullage) in orbit, many of which I have witnessed. It was dramatically brighter than the many boosters I have witnessed, and there apparently was no regular periodicity to the flare-up of the object; it flared only once, during the observation.Note: Both witnesses, who have normal color perception, had the impression that the object had "color" to it, when it flared dramatically. The second witness reported that he thought he witnessed a "purple" tint to the light, during the flare-up, although the color change was subtle.Both witnesses are quite familiar with satellites in the night sky, and both have been witness to over-flights of the ISS, most recently on March 25, 2009, at 20:05 hrs. (Pacific) when both the ISS and Space Shuttle were visible, flying in formation, in the northern sky. They had performed their undocking maneuver at 13:35 hrs., on that date.I remain mystified by this event. If anyone can identify a satellite or ullage that would have been visible from our location, at the time and date indicated, I would be most grateful if you would please submit that information to NUFORC, so that they can post that information, to supplement this report.Thank you!