Frequency Variation of the Kilohertz Quasi-periodic Oscillations and the Flux of the Band-limited Noise in Scorpius X-1

TL;DR

This study investigates the relationship between kilohertz QPOs, band-limited noise, and flux variations in Scorpius X-1, revealing different correlations on the horizontal and normal branches, and suggesting distinct origins for these phenomena.

Contribution

It provides new insights into the correlation between kHz QPO frequencies and flux variations on different branches, indicating different physical origins for BLN and NBO in Scorpius X-1.

Findings

Twin kHz QPO frequencies are positively correlated with flux on the HB.

Frequency separation between twin kHz QPOs decreases with flux on BLN time scales.

Flux variations associated with BLN likely originate from mass accretion rate changes.

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 (LMXB) Scorpius X--1 with the observations performed with the {\it Rossi X-Ray Timing Explorer (RXTE)}. We find that the twin kHz QPO frequencies are positively correlated with the flux variations taking place on the BLN time scales on the HB, in contrast to the anti-correlation held on the time scale 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 origins. We also show the evidence that the frequency separation between the twin kHz QPOs decreases with the flux by $2\sim~3$ Hz on the BLN time scales, which is consistent with the trend on the longer time scale that the Z source traces 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.