On the Disappearance of kHz QPOs at High Mass Accretion Rates

For all sources in which the phenomenon of kilo-Hertz quasi-periodic oscillation (kHz QPO) is observed, the QPOs disappear abruptly when the inferred mass accretion rate exceeds a certain threshold. Although the threshold cannot at present be accurately determined (or even quantified) observationally, it is clearly higher for bright Z sources than for faint atoll sources. Here, we propose that the observational manifestation of kHz QPOs requires direct interaction between the neutron star magnetosphere and the Keplerian accretion disk, and that the cessation of kHz QPOs at high accretion rates is due to the lack of such an interact when the Keplerian disk terminates at the last stable orbit but the magnetosphere is pushed further inward. The threshold is, therefore, dependent of the magnetic field strength — the stronger the magnetic field the higher the threshold. This is certainly in agreement with the atoll/Z paradigm, but we argue that it is also generally true, even for individual sources within each (atoll or Z) category. For atoll sources, the kHz QPOs also seem to vanish at low accretion rates. Perhaps, the “disengagement” between the magnetosphere and the Keplerian disk also takes place under such circumstances, due, for instance, to the presence of quasi-spherical advection-dominated accretion flow (ADAF) close to the neutron star. Unfortunately, in this case, the estimation of the accretion rate threshold would require the knowledge of the physical mechanisms that cause the disengagement. If the ADAF is responsible, the threshold is likely dependent of the magnetic field of the neutron star. Subject headings: accretion, accretion disks — binaries: general — relativity — stars: oscillations — X-ray: stars