  An explanation is proposed for the fact that in LMXBs the correlation between
most observable X-ray spectral and timing parameters (such as kHz QPO
frequency) on the one hand, and Lx on the other, while generally good in a
given source on a time scale of hours, is absent both on longer time scales and
between sources. This leads to parallel tracks in plots of such parameters vs.
Lx. Where previous explanations require at least two time-variable independent
parameters, e.g. Mdot through the disk and through a radial inflow, one is in
fact sufficient if the systemic response to time variations in this variable
has both a prompt and a time-averaged component. I explore a scenario in which
most observable spectral and timing parameters to first order depend on disk
accretion rate normalized by its own long-term average rather than on any
individual Mdot; Lx just depends on total Mdot. Thus, parameters can be
uncorrelated to Mdot, yet vary in response to Mdot variations. Numerical
simulations of the model describing the dependence of kHz QPO frequency on Lx,
which observationally is characterized by a striking pattern of parallel tracks
both in individual sources and between sources, reproduce the observations
remarkably well. A physical interpretation involving a radial inflow with a
rate that derives through a time averaging process from the disk accretion
rate, and an inner disk radius that depends on the balance between the
accretion through the disk and the total luminosity seems particularly
promising. The consequences of this idea for our understanding of states and
tracks in LMXBs are discussed, and the applicability of the idea to black-hole
candidates, where the observational situation is more complex, is briefly
addressed.
