FREQUENCY VARIATION OF THE KILOHERTZ QUASI-PERIODIC OSCILLATIONS AND THE FLUX OF THE BAND-LIMITED NOISE IN SCORPIUS X-1

Abstract

We study the kilohertz quasi-periodic oscillations (kHz QPOs) and the band-limited noise (BLN) in the 0.5–16 Hz range observed simultaneously on the horizontal branch (HB) and on the upper normal branch (NB) of the brightest neutron star low-mass X-ray binary Scorpius X-1 with observations performed with the Rossi X-Ray Timing Explorer. We find that the twin kHz QPO frequencies are positively correlated with the flux variations taking place on the BLN timescales on the HB, in contrast to the anticorrelation held on the timescale of the normal-branch oscillation (NBO) on the NB reported previously, suggesting that although they occur in sequence along the color–color tracks, the BLN and the NBO are of different origin. We also present evidence that the frequency separation between the twin kHz QPOs decreases with the flux by 2–3 Hz on the BLN timescales, which is consistent with the trend on the longer timescale that the Z source traces on the HB. This further suggests that the flux variation associated with the BLN originates from the mass accretion rate variation in the disk accretion flow. We discuss the implications of these results for our understanding of the BLN.