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 \pm 4.3 Hz in an average Fourier power spectrum of observations obtained in September 2001 and January 2002 .
The lower kHz QPO frequency varied between 644 Hz and 769 Hz .
In previous observations of this source the peak separation frequency was \sim 250 Hz when the lower kHz QPO frequency was \sim 900 Hz .
Burst oscillations occur in 4U 1636–53 at \sim 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 .