Abstract
Ginga observations of the quasi-periodic oscillations of three low-mass binary X-ray sources, GX5-1 (centroid frequency: 5.5 Hz), X1820-303 (7.5 Hz) and Cyg X-2 (5.2 Hz) are presented. All these low-frequency QPO's were detected when the spectral hardness of the X-ray was positively correlated with the X-ray intensity. The centroid frequency and FWHM of these QPO's are consistent with being constant for all the three sources respectively. The relative amplitude of QPO increases with increasing X-ray energy in a energy range 5–18 keV for all the sources. For Cyg X-2, however, the relative amplitude below 5 keV is larger than that around 5 keV. Furthermore, energy dependent phase shifts as large as 70 m sec were found in the QPO of this source. Both the energy dependent phase shifts and QPO amplitude of Cyg X-2 can be reproduced if we introduce two oscillating parameters with a phase difference of 150 degrees.
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