We report the discovery of kHz fluctuations , including quasi-periodic oscillations ( QPO ) at \sim 330 ~ { } Hz ( 260 - 407 ~ { } Hz ) and \sim 760 ~ { } Hz ( 671 - 849 ~ { } Hz ) and a broadband kHz continuum in the power density spectrum of the high mass X-ray binary pulsar Centaurus X-3 ( [ Jernigan , Klein and Arons 1999 ] ) .
These observations of Cen X-3 were carried out with the Rossi X-ray Timing Explorer ( RXTE ) .
The fluctuation spectrum is flat from mHz to a few Hz , then steepens to f ^ { -2 } behavior between a few Hz and \sim 100 Hz .
Above a hundred Hz , the spectrum shows the QPO features , plus a flat continuum extending to \sim 1200 ~ { } Hz and then falling out to \sim 1800 ~ { } Hz .
These results , which required the co-adding three days of observations of Cen X-3 , are at least as fast as the fastest known variations in X-ray emission from an accreting compact object ( kHz QPO in LMXB sources ) and probably faster since extension to \sim 1800 ~ { } Hz is indicated by the most likely parameterization of the data .

Multi-dimensional radiation hydrodynamics simulations of optically thick plasma flow onto the magnetic poles of an accreting neutron star show that the fluctuations at frequencies above 100 Hz are consistent with photon bubble turbulence and oscillations ( PBO ) previously predicted ( [ Klein et al . 1996a ] ) to be observable in this source .
We show that previous observations of Cen X-3 constrain the models to depend on only one parameter , the size of the polar cap .
For a polar cap opening angle of 0.25 radians ( polar cap radius \sim 2.5 km and area \sim 20 km ^ { 2 } , for a neutron star radius of 10 km ) , we show that the spectral form above 100 Hz is reproduced by the simulations , including the frequencies of the QPO and the relative power in the QPO and the kHz continuum .
This has resulted in the first model-dependent measurement of the polar cap size of an X-ray pulsar .
The simulations underpredict the overall amplitude of the observed spectrum , which we suggest is the consequence of a 2D axisymmetric simulation of an intrinsically 3D phenomenon .
The power density spectrum of Cen X-3 shows a dramatic decrease above \sim 1000 ~ { } Hz which suggests an optical depth \sim 30 across the accretion mound consistent with effects of radiative diffusion in the simulations .
We identify this decline at high frequency as the first direct evidence of radiative diffusion near the surface of a neutron star ( NS ) .

We suggest the fluctuations observed at frequencies below 100 Hz , whose spectrum has a different form from that of the kHz phenomena , reflect intermittency in the mass transfer mechanism which carries plasma from the accretion disk to field aligned flow onto the neutron star ’ s polar caps .
Using simple estimates based on Rayleigh-Taylor instabilities , possibly modulated by intrinsic disk turbulence , we show that mass transfer in “ blobs ” forming through Rayleigh-Taylor disruption of the disk ’ s inner edge can explain the large amplitude fluctuations required by the spectrum at frequencies f \sim 1 Hz , but only if magnetic pressure in the disk ’ s innermost regions inflates the disk until its scale height is comparable to the magnetosphere ’ s size \sim 4300 km .

The observational results required the development of a procedure for the careful determination of the deadtime effects of the PCA .
This procedure is described in appendix A .
As a consequence of the use of observations of Cyg X-1 for the estimation of deadtime corrections for the observations of Cen X-3 , we have also demonstrated that the black hole Cyg X-1 shows clear evidence of variability up to a frequency of \sim 280 ~ { } Hz .
Also observations of GX 17+2 were used to validate the procedure for deadtime corrections .
This analysis of GX 17+2 clearly indicates the presence of a kHz QPO and the absence of any significant simultaneous kHz continuum .