  Timing properties of black hole X-ray binaries in outburst can be modeled
with mass accretion rate fluctuations propagating towards the black hole. Such
models predict time lags between energy bands due to propagation delays. First
application of a propagating fluctuations model to black hole power spectra
showed good agreement with the data. Indeed, hard lags observed from these
systems appear to be in agreement with this generic prediction. Our PROPFLUC
code allows to simultaneously predict power spectra, time lags, and coherence
of the variability as a function of energy. This was successfully applied to
Swift data on the black hole MAXIJ1659-152, fitting jointly the power spectra
in two energy bands and the cross-spectrum between these two bands. In the
current work, we attempt to to model two high signal to noise Rossi X-ray
Timing Explorer (RXTE) observations of the black hole XTE J1550-564. We find
that neither observation can be adequately explained by the model even when
considering, additionally to previous PROPFLUC versions, different propagation
speeds of the fluctuations. After extensive exploration of model extensions, we
tentatively conclude that the quantitative and qualitative discrepancy between
model predictions and data is generic to the propagating fluctuations paradigm.
This result encourages further investigation of the fundamental hypotheses of
the propagating fluctuations model. We discuss some of these hypotheses with an
eye to future works.
