ASTRONOMY AND ASTROPHYSICS 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 neutron star and black hole binaries at the low/hard spectral state. In the power density spectra of accreting neutron stars with a weak magnetic field a significant power is contained at frequencies close to kHz. At the same time, most Galactic accreting black holes demonstrate a strong decline in the power spectra at the frequencies higher than 10–50 Hz. We propose to use this empirical fact as a method to determine the nature of the compact object in X-ray transients. The X-ray transients that demonstrate significant noise in their X-ray flux at frequencies above ∼ 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 following the model of Inogamov and Sunyaev (1999).