Dependence of kilohertz quasi-periodic oscillation properties on the normal-branch oscillation phase in Scorpius X-1

We analyzed Rossi X-Ray Timing Explorer data of Scorpius X-1, which show ki lohertz quasi-periodic oscillations (QPOs) and the similar to6-8 Hz. normal -branch oscillation (NBO) simultaneously. Using power spectra of 0.03-0.5 s data segments, we find that both the upper kilohertz QPO frequency nu (2) and the ratio of lower to upper kilohertz QPO amplitude are anticorrelated to variations in the X-ray count rate taking place on the NBO timescale. Th e frequency dependence is similar to (but probably weaker than) that found on longer timescales, but the power ratio dependence is opposite to it. A m odel where radiative stresses on the disk material, modulated at the NBO fr equency, lead to changes in nu (2) can explain the data; this implies that some of the NBO flux changes originate from inside the inner disk radius. W e discuss how these findings affect our understanding of kilohertz QPOs and of the low-frequency variability of low-mass X-ray binaries.