Short-term X-ray variability of GX 339-4.

A black hole candidate GX 339 - 4 (4U 1659 - 48) was observed in 1981 and 1982 with the X-ray astronomy satellite Hakucho for more than 50 days in total. In 1981 April and 1982 May, the source was in the X-ray off state (less than about 15 mCrab units), while the optical counterpart was also very faint. Through a substantial part of 1981 June, GX 339 - 4 was in a 0.1-0.2 Crab intensity state with a relatively hard spectrum. On this occasion, it exhibited a persistent, aperiodic intensity variation with about 35 percent rms amplitude on time scales of a fraction of a second to tens of seconds. In the 1-22 keV range, the characteristics of the fluctuation were independent of X-ray energy. In late June of 1981, the source made a transition into a much brighter (about 0.5 Crab intensity) state, with a significant spectral softening. After the transition, the short-term X-ray variation disappeared. The similarity between GX 339 - 4 and Cyg X-1 is discussed.