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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.

Quasi‐periodic oscillations in XTE J1550−564: the rms–flux relation