2003A&A...410..217G

Using Fourier frequency resolved X-ray spectroscopy we study short term spectral variability in luminous LMXBs. With RXTE/PCA observations of 4U 1608-52 and GX 340+0 on the horizontal/normal branch of the color-intensity diagram we show that aperiodic and quasiperiodic variability on ∼second-millisecond 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)∝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 considered range {dot}(M)~(0.1-1){dot}(M)_Edd it depends weakly on the global mass accretion rate and in the limit {dot}(M)~{dot}(M)_Edd is close to Wien spectrum with kT∼2.4keV (in the distant observer's frame). The spectrum of the accretion disk emission is significantly softer and in the 3-20keV range is reasonably well described by a relativistic disk model with a mass accretion rate consistent with the value inferred from the observed X-ray flux.