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The intensity variations of Cyg X-2 and GX 5-1 are analyzed with a Fourier cross-spectral technique making it possible to measure the frequency dependence of the time lags between the intensity variations in two X-ray spectral bands. In the 20-40 Hz frequency range of the quasi-periodic oscillations (QPO), the hard (about 5-18 keV) signal lags the soft (about 1-5 keV) one, while the reverse is true for the associated red noise. The observed time lags range from 0.4 to 8 ms. For similar QPO parameters, the observed lags are much smaller in GX 5-1 than they are in Cyg X-2. The hard lags previously measured in the cross-correlation functions of these sources are now identified as due to lags in the QPO. It is argued that the soft lags now detected in the red noise are not inconsistent with the interpretation of the hard lags as due to inverse Compton scattering. 18 references.

The complex cross-spectra of Cygnus X-2 and GX 5-1