Abstract
We have discovered quasi-periodic X-ray oscillations (QPOs) (30–45 Hz) and low-frequency noise (below 15 Hz) in Cyg X-2 (ref. 1). The centroid frequency of the oscillations is correlated with the source intensity in the same way as in GX 5-1 (refs 2, 3): the frequency increases when the source intensity goes up. Cyg X-2 differs, however, from GX 5-1 (refs 2, 3), where the amplitude of the oscillations and of the low-frequency noise go hand in hand, in that the amplitude of the QPOs decreases while that of the low-frequency noise increases with increasing source intensity. We describe here the spectrum of the persistent emission as the sum of a variable 1.2-keV black-body component, a more or less constant 7 keV thermal bremsstrahlung component and an iron emission line. This is the first report of spectral variations associated with QPOs2–6, which could themselves reflect variations in either the black body or the thermal bremsstrahlung component.
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Hasinger, G., Langmeier, A., Sztajno, M. et al. Quasi-periodic oscillations in the X-ray flux of Cyg X-2. Nature 319, 469–471 (1986). https://doi.org/10.1038/319469a0
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DOI: https://doi.org/10.1038/319469a0
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