KiloHertz quasi – periodic oscillations difference frequency exceeds inferred spin frequency in 4 U 1636 – 53 3

Recent observations of the low–mass X–ray binary 4U 1636–53 with the Rossi X– ray Timing Explorer show, for the first time, a kiloHertz quasi–periodic oscillation (kHz QPO) peak separation that exceeds the neutron star spin frequency as inferred from burst oscillations. This strongly challenges the sonic–point beat frequency model for the kHz QPOs found in low–mass X–ray binaries. We detect two simultaneous kHz QPOs with a frequency separation of 323.3±4.3 Hz in an average Fourier power spectrum of observations obtained in September 2001 and Januari 2002. The lower kHz QPO frequency varied between 644 Hz and 769 Hz. In previous observations of this source the peak separation frequency was ∼250 Hz at a lower kHz QPO frequency of ∼900 Hz. Burst oscillations occur in 4U 1636–53 at ∼581 Hz and possibly at half that frequency (290.5 Hz). This is the first source where the peak separation frequency is observed to change from less than (half) the burst oscillation frequency to more than that. This observation contradicts all previously formulated implementations of the sonic–point beat frequency model except those where the disk in 4U 1636–53 switches from prograde to retrograde. We discuss possible ways out for the sonic–point beat frequency model and other models for the kHz QPOs.