Astron. Astrophys. 323, 399-414 (1997)
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
( 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
- The overall intensity changes with a factor of
1.34 between the medium and the high overall
- 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
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
- Detection of five bursts in the Ginga data, which do not show the
characteristics of bona fide type I bursts.
© European Southern Observatory (ESO) 1997
Online publication: June 5, 1998