Quasi-periodic oscillations in XTE J1550−564: the rms-flux relation.

We present an analysis of the short time-scale variations in the properties of the strong (type 'C') quasi-periodic oscillation (QPO) 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 time-scales shorter than ks. 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 Hz, through no correlation, to a strong negative correlation when the peak frequency is above 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 Hz become constant. [ABSTRACT FROM AUTHOR]