1999A&A...343..197V

We have measured the phase differences between X-ray intensity variations in different energy bands in data taken with Ginga of the Z source GX5-1 in its normal branch. We apply a χ² optimal extraction technique to measure the phase-delay spectrum of the normal-branch quasi-periodic oscillations (QPO) and find a jump at 3.5keV. X-ray intensity variations at the QPO frequency in energy channels below 3.5keV are in phase with one another. The same is true of energy channels above 6keV. However, intensity variations in energy channels above 6keV lag those in channels below 3.5keV by ∼150°. We see no minimum in the fractional rms amplitude of the quasi-periodic oscillations at the jump energy, in contrast to what is seen in the Z source Cyg X-2, but because of limited statistics we cannot rule out such a minimum. We discuss our findings in the context of a radiation-hydrodynamics model for the normal-branch quasi-periodic oscillations.