  We report on RXTE observations of the microquasar XTE J1550 during a ~70 day
outburst in April-June 2000. We focus here on the temporal properties of the
source and study the behavior of low frequency (0.1-10 Hz) quasi periodic
oscillations (LFQPO), which seem to be of different types. We focus on the
so-called type C (according to the classification of Remillard and
collaborators), which corresponds to a strong 0.1-6 Hz LFQPO, found to be
present during at least 17 observations. We find that the frequency of the QPO
is better correlated with the soft X-ray (< 7 keV) flux than with the hard flux
(> 7 keV). If soft X-rays represent the behavior of an accretion disk, the
relation shows that the disk may set the LFQPO frequency. In two cases, the
identification of the type of QPO is not straightforward. If the QPOs in those
two cases are type A (or B), then we may be seeing the QPO type alternate
between type C and type A (or B), and this may represent some rapid changes in
the physical properties of the accretion flow, before the system stabilizes and
slowly decays toward the end of the outburst. On the other hand, if all the
QPOs are of type C, we may be observing an inversion in the frequency vs. flux
relation, similar to that seen in GRO J1655-40. We discuss the QPO behavior in
the framework of theoretical models.
