Kilohertz Quasi-periodic Oscillation Peak Separation Is Not Constant in Scorpius X-1

We report on a series of twenty ~10^5 c/s, 0.125 msec time-resolution RXTEobservations of the Z source and low-mass X-ray binary Scorpius X-1. Twinkilohertz quasi-periodic oscillation (QPO) peaks are obvious in nearly allobservations. We find that the peak separation is not constant, as expected insome beat-frequency models, but instead varies from ~310 to ~230 Hz when thecentroid frequency of the higher-frequency peak varies from ~875 to ~1085 Hz.We detect none of the additional QPO peaks at higher frequencies predicted inthe photon bubble model (PBM), with best-case upper limits on the peaks' powerratio of 0.025. We do detect, simultaneously with the kHz QPO, additional QPOpeaks near 45 and 90 Hz whose frequency increases with mass accretion rate. Weinterpret these as first and second harmonics of the so-calledhorizontal-branch oscillations well known from other Z sources and usuallyinterpreted in terms of the magnetospheric beat-frequency model (BFM). Weconclude that the magnetospheric BFM and the PBM are now unlikely to explainthe kHz QPO in Sco X-1. In order to succeed in doing so, any BFM involvingthe neutron star spin (unseen in Sco X-1) will have to postulate at least oneadditional unseen frequency, beating with the spin to produce one of the kHzpeaks.