Ginga observations of Cygnus X-2

WehaveanalysedallavailableX-raydataonthelow- mass X-ray binary Cygnus X-2 obtained with the Ginga satel- lite.Adetailedanalysisofthespectralandfasttimingbehaviour of these 4 years of data provides new insights in the behaviour of this Z source. We conrm the previously observed recurrent patterns of behaviour in the X-ray colour-colour and hardness- intensity diagrams consisting of shifts and shape changes in the Z track. However, we nd a continuous range of patterns rather than a discrete set. The source behaviour in the diagrams is cor- related with overall intensity, which varied by a factor of 1.34 in the Ginga data. We nd that when the overall intensity in- creases, the mean velocity and acceleration of the motion along thenormalbranchoftheZtrackincrease,aswellasthewidthof the normal branch in the hardness-intensity diagram. Contrary to previous results we nd that, during different observations, when the source is at the same position in the normal branch of the Z track the rapid X-ray variability differs signicantly. During the Kuulkers et al. (1996a) \medium" level, a normal branch quasi-periodic oscillation is detected, which is not seen during the \high" overall intensity level. Also, during the high overall intensity level episodes the very-low frequency noise on the lower normal branch is very strong and steep, whereas during the medium overall intensity level episodes this noise component at the same position in the Z track is weak and less steep. The explanation of the different overall intensity levels withaprecessingaccretiondiskisdifculttoreconcilewithour data.Furthermore,wefoundthatthefrequencyofthehorizontal branch quasi-periodic oscillation decreases when Cygnus X-2 enters the upper normal branch, giving a model dependent up- per limit on the magnetic eld strength at the magnetic equator of 8:5 10 9 G. We also report ve bursts, with durations between two and eight seconds, whose occurrence seems to be uncorrelated with location in the Z track, overall intensity level or orbital phase. The burst properties indicate that they are not regular type I bursts.