Fourier power spectra at high frequencies: a way to distinguish a neutron star from a black hole

We analyzed the power density spectra of a sample of 9 neutron star and 9 b lack hole binaries in the low/hard spectral state. In the power density spe ctra of accreting neutron stars with a weak magnetic field a significant po wer is contained at frequencies close to one kHz. At the same time, most Ga lactic accreting black holes demonstrate a strong decline in the power spec tra at the frequencies higher than 10-50 Hz. We propose to use this empiric al fact as a method to distinguish the accreting neutron stars from black h oles in X-ray transients. The X-ray transients that demonstrate significant noise in their X-ray flux at frequencies above similar to 500 Hz should be considered neutron stars. We propose to explain the observed difference as a result of the existence of a radiation dominated spreading layer on the neutron star surface (Inogamov & Sunyaev 1999). The possible very high freq uency variabilities of this layer are discussed.