We have measured the phase-delay and rms amplitude spectra of Cyg X-2 and S
co X-l. Using EXOSAT data from the normal branch of Cyg X-2 we confirm earl
ier (Ginga) results, showing that at an energy near 6 keV there is both a m
inimum in the quasi-periodic oscillations QPO rms amplitude spectrum and a
150 degrees phase jump in the QPO phase-delay spectrum.
Surprisingly, using EXOSAT and Ginga data, we find no evidence for a phase
jump of this kind in the phase-delay spectrum of Sco X-l on either the norm
al or flaring branch. Upper limits (90% confidence) of 42 degrees can be se
t on any phase jump in the energy range 2-10 keV on the normal branch, and
88 degrees on the flaring branch. The QPO rms amplitude spectrum of Sco X-l
increases steeply with energy on both the normal and flaring branches. The
se results suggest that the X-ray spectrum pivots about an energy of less t
han or similar to 2keV or greater than or similar to 10 keV or that normal
branch QPO of Sco X-l does not have a pivoting spectrum. We discuss the imp
lications of these results in terms of the radiation-hydrodynamic model for
normal branch QPO.