We show that aperiodic and quasiperiodic variability of bright LMXBs – atoll and Z- sources , on \sim sec – msec time scales is caused primarily by variations of the luminosity of the boundary layer .
The emission of the accretion disk is less variable on these time scales and its power density spectrum follows P _ { disk } ( f ) \propto f ^ { -1 } law , contributing to observed flux variation at low frequencies and low energies only .
The kHz QPOs have the same origin as variability at lower frequencies , i.e .
independent of the nature of the ” clock ” , the actual luminosity modulation takes place on the neutron star surface .
The boundary layer spectrum remains nearly constant in the course of the luminosity variations and is represented to certain accuracy by the Fourier frequency resolved spectrum .
In the investigated range of \dot { M } \sim ( 0.1 - 1 ) \dot { M } _ { Edd } it depends weakly on the global mass accretion rate and in the limit \dot { M } \sim \dot { M } _ { Edd } is close to Wien spectrum with kT \sim 2.4 keV .
Its independence on the global value of \dot { M } lends support to the theoretical suggestion by Inogamov & Sunyaev ( 11 ) that the boundary layer is radiation pressure supported . Based on the knowledge of the boundary layer spectrum we attempt to relate the motion along the Z-track to changes of physically meaningful parameters .
Our results suggest that the contribution of the boundary layer to the observed emission decreases along the Z-track from conventional \sim 50 \% on the horizontal branch to a rather small number on the normal branch .
This decrease can be caused , for example , by obscuration of the boundary layer by the geometrically thickened accretion disk at \dot { M } \sim \dot { M } _ { Edd } .
Alternatively , this can indicate significant change of the structure of the accretion flow at \dot { M } \sim \dot { M } _ { Edd } and disappearance of the boundary layer as a distinct region of the significant energy release associated with the neutron star surface .