  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.
