Astron. Astrophys. 323, 399-414 (1997)

Ginga observations of Cygnus X-2

R.A.D. Wijnands ¹, M. van der Klis ¹, E. Kuulkers ², K. Asai ³ and G. Hasinger <sup>4

1</sup> Astronomical Institute "Anton Pannekoek", University of Amsterdam and Center for High Energy Astrophysica (CHEAF), Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
² ESA/ESTEC, Astrophysics Division (SA), P.O. Box 229, 2200 AG Noordwijk, The Netherlands
³ Institute of Space and Astronautical Science, 1-1 Yoshinodai 3-chome, Sagamihara, Kanagawa 229, Japan
⁴ Astrophysikalisches Institut Potsdam, An der Sternwarte 16, D-14482 Potsdam, Germany

Received 1 October 1996 / Accepted 20 December 1996

Abstract

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 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 consisting of shifts and shape changes in the Z track. However, we find a continuous range of patterns rather than a discrete set. The source behaviour in the diagrams is correlated with overall intensity, which varied by a factor of 1.34 in the Ginga data. We find that when the overall intensity increases, the mean velocity and acceleration of the motion along the normal branch of the Z track increase, as well as the width of the normal branch in the hardness-intensity diagram. Contrary to previous results we find 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 significantly. 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 with a precessing accretion disk is difficult to reconcile with our data. Furthermore, we found that the frequency of the horizontal branch quasi-periodic oscillation decreases when Cygnus X-2 enters the upper normal branch, giving a model dependent upper limit on the magnetic field strength at the magnetic equator of G. We also report five 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.

Key words: accretion, accretion disks stars: individual: Cyg X-2 stars: neutron X-rays: stars

Send offprint requests to: R.A.D. Wijnands (rudy@astro.uva.nl)

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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