We report on a series of twenty \sim 10 ^ { 5 } c/s , 0.125 msec time-resolution RXTE observations of the Z source and low-mass X-ray binary Scorpius X-1 .
Twin kilohertz quasi-periodic oscillation ( QPO ) peaks are obvious in nearly all observations .
We find that the peak separation is not constant , as expected in some beat-frequency models , but instead varies from \sim 310 to \sim 230 Hz when the centroid frequency of the higher-frequency peak varies from \sim 875 to \sim 1085 Hz .
We detect none of the additional QPO peaks at higher frequencies predicted in the photon bubble model ( PBM ) , with best-case upper limits on the peaks ’ power ratio of 0.025 .
We do detect , simultaneously with the kHz QPO , additional QPO peaks near 45 and 90 Hz whose frequency increases with mass accretion rate .
We interpret these as first and second harmonics of the so-called horizontal-branch oscillations well known from other Z sources and usually interpreted in terms of the magnetospheric beat-frequency model ( BFM ) .
We conclude that the magnetospheric BFM and the PBM are now unlikely to explain the kHz QPO in Sco X-1 .
In order to succeed in doing so , any BFM involving the neutron star spin ( unseen in Sco X-1 ) will have to postulate at least one additional unseen frequency , beating with the spin to produce one of the kHz peaks .