MECHANISMS FOR HIGH-FREQUENCY QUASI-PERIODIC OSCILLATIONS IN NEUTRON-STAR AND BLACK-HOLE BINARIES

We explain the millisecond variability detected by Rossi X-Ray Timing Explorer (RXTE) in the X-ray emission from a number of low-mass X-ray binary systems (Sco X-1, 4U 1728-34, 4U 1608-522, 4U 1636-536, 4U 0614 +091, 4U 1735-44, 4U 1820-30, GX 5-1) in terms of dynamics of the cent rifugal barrier, a hot boundary region surrounding a neutron star (NS) . We demonstrate that this region may experience the relaxation oscill ations and that the displacements of a gas element both in radial and vertical directions occur at the same main frequency, of order of the local Keplerian frequency. We show the importance of the effect of a s plitting of the main frequency produced by the Coriolis force in a rot ating disk for the interpretation of a spacing between the quasi-perio dic oscillation (QPO) peaks. We estimate a magnitude of the splitting effect and present a simple formula for the whole spectrum of the spli t frequencies. It is interesting that the first three lowest order ove rtones (corresponding to the azimuthal numbers m = 0, -1, and -2) fall in the range of 200-1200 Hz and match the kHz QPO frequencies observe d by RXTE. Similar phenomena should also occur in black hole (BH) syst ems, but, since the QPO frequency is inversely proportional to the mas s of a compact object, the frequency of the centrifugal-barrier oscill ations in the BH systems should be a factor of 5-10 lower than that fo r the NS systems. The X-ray spectrum formed in this region is a result of upscattering of a soft radiation (from a disk and an NS surface) o ff relatively hot electrons in the boundary layer. The typical size of the emission region should be 1-3 km, which is consistent with the ti me-lag measurements. We also briefly discuss some alternative QPO mode ls, including the possibility of acoustic oscillations in the boundary layer, the proper stellar rotation, and g-mode disk oscillations.