Quasi-periodic Oscillations in XTE J1550-564: The rms-flux relation

TL;DR

This paper reports the first detection of a variable rms-flux relation in a QPO in XTE J1550-564, revealing a correlation with QPO frequency and suggesting a filtering effect influencing QPO amplitude.

Contribution

It introduces the first observation of rms-flux dependence in a QPO and explores its relation to QPO frequency, providing new insights into QPO behavior.

Findings

RMS-flux relation varies with QPO frequency.

Negative rms-flux relation observed above 6 Hz.

Filtering effects may influence QPO amplitude at high frequencies.

Abstract

We present an analysis of the short timescale variations in the properties of the strong (type "C") quasi-periodic oscillation observed in XTE J1550-564 during its 1998 outburst. In particular, the QPO shows a correlation between absolute rms amplitude and mean source flux over timescales shorter than ~ 3 ksec. A linear rms-flux relation has been observed to be a common property of broad-band noise but here we report the first detection of rms-flux dependence in a QPO. The gradient of the rms-flux relation is correlated with the QPO peak frequency: from a strong positive correlation when the QPO peak frequency is below ~ 4 Hz, through no correlation, to a strong negative correlation when the peak frequency is above 6 Hz. This is the first time a negative short term rms-flux relation has been observed in any component of the power spectrum. Previous work on both the broad-band noise and QPOs in a range of sources have suggested the presence of a filter reducing the amplitude of QPOs with increasing frequency. We attempt to remove the possible effects of this filter and find that the previously negative rms-flux relations above ~ 5 Hz become constant.