Discovery of kHz QPOs and shifted frequency correlations in the accreting millisecond pulsar XTE J1807--294

We report the discovery of twin kHz QPOs in the X ray flux of XTE J1807--294, the fourth accreting millisecond pulsar (AMP). This is the second AMP exhibiting twin kHz QPOs. In contrast to the first case, SAX J1808.4--3658, the frequency separation $Δν$ between the kHz QPOs is consistent with the pulse frequency (190.6 Hz), not with half that value, confirming for the first time from pulsation measurements the inference, based on burst oscillations, that 'slow rotators' (spin frequency less than 400 Hz) have $Δν$ approximately equal to the spin frequency. While the QPOs move in frequency together over a range of more than 200 Hz, $Δν$ remains constant with an average value of 205$\pm$6 Hz. Variability components were found in the 5--130 Hz range similar to those seen in other LMXBs. The correlations between the QPO and noise frequencies are also similar to those in other sources, but shifted by a factor of 1.59 in kHz QPO frequencies, similar to the factor 1.45 shift found for SAX J1808.4--3658. Our results argue in favour of a spin-related formation mechanism for twin kHz QPOs and against a spin-related cause of the shift in the frequency correlations.