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 kilohertz 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. The frequency dependence is similar to (but probably weaker than) that found on longer timescales, but the power ratio dependence is opposite to it. A model where radiative stresses on the disk material, modulated at the NBO frequency, 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. We discuss how these findings affect our understanding of kilohertz QPOs and of the low-frequency variability of low-mass X-ray binaries.