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

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7. Conclusion

We observed Cygnus X-2 over a broad range of intensities. The source alternated between the known medium and high overall intensity levels and also sometimes in a state in between these two levels, both in intensity and other source characteristics. We found new correlations between several characteristics of the source and the overall intensity level:

  • The velocity and acceleration along the normal branch increase when the overall intensity increases. During the high intensity level Cygnus X-2 moves faster and more irregularly up and down the normal branch than during lower levels.
  • The width of the normal branch in the hardness-intensity diagram increases when the overall intensity increases.
  • The very-low frequency noise near the soft vertex increases in amplitude and in steepness when the overall intensity increases.
  • The normal branch quasi-periodic oscillation is not detectable ([FORMULA] 0.9 % rms) near the soft vertex when Cygnus X-2   is in the high level. During the other levels a NBO can be easily detected at a level of 1-2.5 % (rms).

The following correlations were already known and are confirmed by our analysis:

  • The overall intensity changes with a factor of [FORMULA] 1.34 between the medium and the high overall intensity level.
  • The shape of the Z-track in the colour-colour diagram and hardness-intensity diagram changes when the overall intensity changes. In the colour-colour diagram the horizontal and the flaring branch become more horizontal when the overall intensity increases. When the source enters the flaring branch during the medium overall intensity level the intensity first increases and later decreases. When the source enters the flaring branch during the high overall intensity level the intensity immediately starts to decrease.
  • The whole Z-track shifts to softer colours when the overall intensity increases.

We also found several other previously unreported phenomena:

  • Clear detection of overall intensity level episodes in-between the medium and the high overall intensity level, indicating that the different overall intensity levels are part of a continouos range instead of a discrete set.
  • The frequency of the horizontal branch quasi-periodic oscillation decreases down the NB, giving a model dependent upper limit on the magnetic field strength at the magnetic equator of [FORMULA] G.
  • Detection of five bursts in the Ginga data, which do not show the characteristics of bona fide type I bursts.
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© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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