GINGA OBSERVATIONS OF CYGNUS X-2

We have analysed all available X-ray data on the low-mass X-ray binary Cygnus X-2 obtained with the Ginga satellite. A detailed analysis of the spectral and fast timing behaviour of these 4 years of data provid es new insights in the behaviour of this Z source. We confirm the prev iously observed recurrent patterns of behaviour in the X-ray colour-co lour and hardness-intensity diagrams consisting of shifts and shape ch anges in the Z track. However, we find a continuous range of patterns rather than a discrete set. The source behaviour in the diagrams is co rrelated with overall intensity, which varied by a factor of 1.34 in t he Ginga data. We find that when the overall intensity increases, the mean velocity and acceleration of the motion along the normal branch o f the Z track increase, as well as the width of the normal branch in t he hardness-intensity diagram. Contrary to previous results we find th at, during different observations, when the source is at the same posi tion in the normal branch of the Z track the rapid X-ray variability d iffers significantly. During the Kuulkers et al. (1996a) ''medium'' le vel, 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 sa me position in the Z track is weak and less steep. The explanation of the different overall intensity levels with a precessing accretion dis k is difficult to reconcile with our data. Furthermore, we found that the frequency of the horizontal branch quasi-periodic oscillation decr eases when Cygnus X-2 enters the upper normal branch, giving a model d ependent upper limit on the magnetic field strength at the magnetic eq uator of similar to 8.5 x 10(9) G. We also report five bursts, with du rations between two and eight seconds, whose occurrence seems to be un correlated with location in the Z track, overall intensity level or or bital phase. The burst properties indicate that they are not regular t ype I bursts.