Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996a%26a...311..197k&link_type=abstract
Astronomy and Astrophysics, v.311, p.197-210
Astronomy and Astrophysics
Astrophysics
54
Accretion, Accretion Disks, Instrumentation: Detectors, Binaries: Close, Stars: Individual: Cygx-2, Stars: Neutron, X-Rays: Stars
Scientific paper
We analyzed the X-ray spectral variations of the low-mass X-ray binary CygX-2 on time scales longer than one day using EXOSAT data. We describe these variations in terms of the shifts and shape changes of the "Z" pattern in X-ray colour-colour and hardness-intensity diagrams. CygX-2 varies between three intensity "levels": "high", "medium" and "low". During the high level episode the Z pattern is shifted to higher overall intensity with respect to the pattern of the medium level episode. There are also differences in the shape of the Z pattern between the high and medium level. During the medium level episode both the colour-colour diagram and hardness intensity diagram show a clear flaring branch. In the upper part of the flaring branch of this episode the intensity drops while the colours increase (`colour-dependent dip'). During one of the high level episodes a branch is seen in the hardness-intensity diagram, but not in the colour-colour diagram, which we interpret as flaring branch behaviour. In this branch the intensity drops while the colours remain the same (`colour-independent dip'). During a low level episode, when the source showed the lowest overall intensity as compared with all other observations, no Z shape was seen but only one large, curved branch due to flares on time scales of 300-500s which are larger at high energies. Previously reported data from other satellites also show this behaviour. We conclude that in total, six high, eleven medium, and three low level episodes have been observed for which sufficient colour-data were available. In total, more than six low level episodes occurred. All low level episodes happened between orbital phases 0.8 and 0.2 (where phase 0.0 is the X-ray source superior conjunction), suggesting that the view of the inner disk was partly obscured, by e.g. the secondary or the mass transfer stream. We ascribe the occurrence of the high, medium and low intensity levels to a component in the system that recurrently obscures (part of) the emitting regions and suggest that it is observable only in sources with high inclinations. We investigate whether the occurrence of the three levels is periodic as would be expected from a precessing disk.
Kuulkers Eric
van der Klis Michael
Vaughan Brian A.
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