The phase-delay spectrum of GX 5-1 on its normal branch

We have measured the phase differences between Xray intensity variations in different energy bands in data taken with Ginga of the Z source GX 5-1 in its normal branch. We apply a chi(2) optimal extraction technique to measur e the phase-delay spectrum of the normal-branch quasi-periodic oscillations (QPO) and find a jump at 3.5 keV. X-ray intensity variations at the QPO fr equency in energy channels below 3.5 keV are in phase with one another. The same is true of energy channels above 6 keV. However, intensity variations in energy channels above 6 keV lag those in channels below 3.5 keV by simi lar to 150 degrees. We see no minimum in the fractional rms amplitude of th e quasiperiodic 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-h ydrodynamics model for the normal-branch quasi-periodic oscillations.