Fig. 5
Temporal behaviour of the soft X-ray luminosity with 1σ deviation as observed by several X-ray missions between 1990 and 2011. ROSAT PSPC data are plotted as light-blue filled circles; ROSAT HRI data are depicted as navy-blue filled circles. The red circles represent XMM-Newton RGS observations. Plotted as a thick black curve is sinusoidal fit to the X-ray data with an optical-cycle period of ≈17 years.
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