Kilohertz quasi-periodic oscillation peak separation is not constant in the atoll source 4U 1608-52

We present new Rossi X-Ray Timing Explorer observations of the low-mass X-r ay binary 4U 1608 - 52 during the decay of its 1998 outburst. We detect, by a direct fast Fourier transform method, the existence of a second kilohert z quasi-periodic oscillation (kHz QPO) in its power density spectrum, which was previously only seen by means of the sensitivity-enhancing "shift and add" technique. This result confirms that 4U 1608-52 is a twin kHz QPO sour ce. The frequency separation between these two QPOs decreased significantly , from 325.5 +/- 3.4 to 225.3 +/- 12.0 Hz, as the frequency of the lower kH z QPO increased from 470 to 865 Hz, in contradiction with a simple beat-fre quency interpretation. This change in the peak separation of the kHz QPOs i s closely similar to that previously seen in Scorpius X-1 but takes place a t a 10 times lower average luminosity. We discuss this result within the fr amework of models that have been proposed for kHz QPO. Beat-frequency model s where the peak separation is identified with the neutron star spin rate, as well as the explanations previously proposed to account for the similar behavior of the QPOs in Sco X-1, are strongly challenged by this result.