Multi-peaked distributions of kHz QPO frequencies in bright neutron star low-mass X-ray binaires

We have analyzed all published frequencies, v1 and v2, of the twin kiloHertz quasi-periodic oscillations (kHz QPOs) in bright neutron star low-mass X-ray binaries. The two frequencies are well correlated but, contrary to recent suggestions, the frequency-frequency correlation is significantly different from a v2 =(3/2)v1 relation. To check whether, although not following the 3/2 relation, the QPO frequencies cluster around the region where v2/v1=3/2, we analyze expanded data sets of Sco X-1 and four other sources: 4U 1608-52, 4U 1636-53, 4U 1728-34 and 4U 1820-30. Surprisingly, we find that in all five source the distributions of the kHz QPO frequencies shows multiple peaks. Within errors these peaks can all be associated with v2/v1 ratios of small integers, 3/2, 4/3, 7/5, and 9/7, not all ratios appearing in all sources. Importantly, we also show that these peaks have no analogy in the distribution of points in the spectral color-color diagrams of these sources. We discuss our results in the framework of recently proposed QPO models and suggest that a resonance model, based on the three basic general relativistic frequencies of a test particle around a compact object, may be able to explain the observed frequency distributions.