Simultaneous measurements of X-ray luminosity and kilohertz quasi-periodicoscillations in low-mass X-ray binaries

We measure simultaneously the properties of the energy spectra and the freq uencies of the kilohertz quasi-periodic oscillations (QPOs) in 15 low-mass X-ray binaries covering a wide range of X-ray luminosities. In each source, the QPO frequencies cover the same range of approximately 300-1300 Hz, alt hough the sources differ by 2 orders of magnitude in their X-ray luminositi es (as measured from the unabsorbed 2-50 keV flux). So the X-ray luminosity does not uniquely determine the QPO frequency. This is difficult to unders tand since the evidence from individual sources indicates that the frequenc y and luminosity are very well correlated at least over short timescales. P erhaps beaming effects or bolometric corrections change the observed lumino sities, or perhaps part of the energy in mass accretion is used to power ou t 8ows, thus reducing the energy emitted in X-rays. It is also possible tha t the parameters of a QPO model are tuned in such a way that the same range of frequencies appears in all sources. Different modes of accretion may be involved (for example, disk and radial), or multiple parameters may conspi re to yield the same frequencies.