The atoll source states of 4U 1608-52

We have studied the atoll source 4U 1608-52 using a large data set obtained with the Rossi X-Ray Timing Explorer. We find that the timing properties of 4U 1608-52 are almost exactly identical to those of the atoll sources 4U 0614+09 and 4U 1728-34 despite the fact that contrary to these sources 4U 1608-52 is a transient covering 2 orders of magnitude in luminosity. The frequencies of the variability components of these three sources follow a universal scheme when plotted versus the frequency of the upper kilohertz quasi-periodic oscillation, suggesting a very similar accretion flow configuration. If we plot the Z sources on this scheme, only the lower kilohertz quasi-periodic oscillation and horizontal branch oscillation follow identical relations. Using the mutual relations between the frequencies of the variability components, we tested several models: the transition layer model, the sonic point beat frequency model, and the relativistic precession model. None of these models described the data satisfactory. Recently, it has been suggested that the atoll sources (among them 4U 1608-52) trace out similar three-branch patterns as the Z sources in the color-color diagram. We have studied the relation between the power spectral properties and the position of 4U 1608-52 in the color-color diagram and conclude that the timing behavior is not consistent with the idea that 4U 1608-52 traces out a three-branched Z shape in the color-color diagram along which the timing properties vary gradually, as Z sources do.