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 ?3 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 ?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.