Ginga observations of Cygnus X-2

Abstract

We analysed all X-ray data on the low-mass X-ray binary Cygnus X-2 obtained with Ginga. The analysis of the spectral and fast timing behaviour of these 4 years of data provides new insights in the behaviour of this Z source. We confirm the previously observed recurrent patterns of behaviour in the X-ray colour-colour and hardness-intensity diagrams (HIDs) consisting of shifts and shape changes in the Z track. The source behaviour is correlated with overall intensity. When the overall intensity increases, the mean velocity of the motion along the normal branch (NB) of the Z track increase, as well as the width of the NB in the HID. Contrary to previous results we find that, during different observations, when the source is at the same position in the NB of the Z track the rapid X-ray variability differs significantly. During the Kuulkers et al. (1996) ``medium'' level, a normal branch quasi-periodic oscillation (QPO) is detected, which is not seen during the ``high'' overall intensity level. During the high overall intensity level episodes the very-low frequency noise on the lower NB is very strong, whereas during the medium overall intensity level episodes this noise component at the same position in the Z track is much weaker. The explanation of the different overall intensity levels with a precessing accretion disk is difficult to reconcile with our data. Furthermore, the frequency of the horizontal branch QPO decreases when Cyg X-2 enters the upper NB, giving a model dependent upper limit on the magnetic field strength at the magnetic equator of ~8.5x10^9 G. We also report 5 bursts, with durations of ~5 s, whose occurrence are 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.