Boundary layer emission and Z-track in the color–color diagram of luminous LMXBs

Abstract

Aims. We explore the accretion disk and boundary layer emission in bright neutron star LMXBs and their dependence on the mass accretion rate. Methods. We used Fourier-frequency resolved spectroscopy of the archival RXTE data. Results. We demonstrate that Fourier-frequency resolved spectra of atoll and Z-sources are identical, despite significant differences in their average spectra and luminosity (by a factor of ∼10−20). This result fits in the picture we suggested earlier, namely that the f > 1 Hz variability in luminous LMXBs is primarily due to variations in the boundary layer luminosity. In this picture the frequency-resolved spectrum equals the boundary layer spectrum, which therefore can be straightforwardly determined from the data. This boundary layer spectrum is approximated well by the saturated Comptonization model, and its high energy cut-off follows a kT ≈ 2.4 keV blackbody. Its independence from the global mass-accretion rate lends support to a theoretical suggestion, that the boundary layer is supported by radiation pressure. With this assumption we constrain the gravity on the neutron star surface, its mass and radius. Equipped with 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 ∼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 thick accretion disk at u M ∼ u MEdd. Alternatively, this can indicate a significant change in the structure of the accretion flow at u M ∼ u MEdd and the disappearance of the boundary layer as a distinct region of the significant energy release associated with the neutron star surface.