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

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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 4

1 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 [FORMULA] 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)

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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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