Dependence of the Frequency of the Kilohertz Quasi-periodic Oscillationson X-Ray Count Rate and Colors in 4U 1608-52

We present new results based on observations carried out with the Rossi X-ray Timing Explorer during the decay of an outburst of the low-mass X-ray binary (LMXB) and atoll source 4U 1608-52. Our results appear to resolve, at least in 4U 1608-52, one of the long-standing issues about the phenomenology of the kilohertz quasi-periodic oscillations (kHz QPOs), namely the lack of a unique relation between the frequency of the kHz QPOs and the X-ray flux. We show that despite its complex dependence on the X-ray flux, the frequency of the kHz QPOs is monotonically related to the position of the source in the color-color diagram. Our findings strengthen the idea that, as in the case of Z sources, in the atoll sources the X-ray flux is not a good indicator of Ṁ and that the observed changes in the frequency of the kHz QPOs in LMXBs are driven by changes in Ṁ. These results raise some concern about the recently reported detection of the orbital frequency at the innermost stable orbit in 4U 1820-30.