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 χ2 optimal extraction technique to measure the phasedelay spectrum of the normal-branch quasi-periodic oscillations (QPO) and find a jump at 3.5 keV. X-ray intensity variations at the QPO frequency 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 ∼150◦. We see no minimum in the fractional rms amplitude of the 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-hydrodynamics model for the normalbranch quasi-periodic oscillations.