Spectral Signatures of KiloHertz Quasi-Periodic Oscillations from Accreting Neutron Stars

Correlations discovered between millisecond timing properties and spectral properties in neutron star x-ray binaries are described and then interpreted in relation to accretion flows in the systems. Use of joint timing and spectral observations to test for the existence of the marginally stable orbit, a key prediction of strong field general relativity, is described and observations of the neutron star x-ray binary 4U1820-303 which suggest that the signature of the marginally stable orbit has been detected are presented.