  We follow the evolution of a low frequency QPO during the 2000 outburst of
the microquasar XTE J1550-564, which was found to be present in the PCA energy
range (2-65 keV) in 19 of 43 observations. The frequency of the QPO varies from
0.1 Hz to 6 Hz, and appears to follow the evolution of the soft
  X-ray flux. If we assume the soft X-rays represent the behavior of an
accretion disk, the relation indicates that this low frequency QPO is linked to
the accretion disk. We show that the non-trivial relation between the QPO
frequency and the soft flux may be as expected from the Accretion Ejection
  Instability (AEI), when the disk approaches its last stable orbit.
Furthermore, the energy dependence of the QPO may indicate the presence of a
hot spot rotating in the disk as predicted by the AEI.
