Mass determinations and X-ray energy spectral analyses are among the methods used to distinguish between the types of compact objects present in X-ray binary systems. We test a method of distinguishing between neutron stars and black holes proposed by Sunyaev and Revnivtsev where power density spectra are used, particularly in the 500-1000Hz range. Sunyaev and Revnivtsev found that only neutron stars appear to have significant power in this frequency range. We apply this criterion to 12 X-ray binary systems (six neutron stars and six black holes) using USA data and cannot reproduce Sunyaev and Revnivtsev's result. The reason for this discrepancy is most likely a USA instrumental effect which manifests itself as excess power in the frequency range of interest. Future work on correcting this problem should provide more accurate analyses that may yield a different result.

One of the remarkable phenomena, characterizing both Galactic and extra-Galactic Xray binary systems, is the substantial variability of a photon ux, detectable in a very broad range of timescales. For instance, the accretion ow near a black hole event horizon can produce X-ray variability on a millisecond timescale. At the same time aperiodic changes from the extended accretion disk formed around the same black hole can occur on timescales of order of several months to years. A complex structure, involving high and low frequency nearly periodic oscillations and aperiodic features, observed in X-ray lightcurves, is the subject of intensive studies. The characteristic quantities, extracted from temporal analysis, carry speci c physical meaning and contain direct observational information about dynamics of the accreting X-ray source. It is the established fact that X-ray spectral and timing properties are tightly correlated. Combined together, the photon energy spectrum and the power density spectrum analyses, form a powerful framework that brings up the complete (in the energy/space domain) picture of the physical processes at work in the accreting system. Simultaneous study of spectral and timing characteristics allows for comprehensive probing of the geometry of accretion ows, reliable identification of the type of an X-ray source (black hole vs neutron star), constraining mass, size, and spin of accreting stellar-mass compact objects. Up until now there is no self-consistent physical model of the formation and evolution of the X-ray variability. This leaves a relative freedom in interpretation of the characteristic quantities obtained from the timing analysis. The current work aims at development of the physical alternative to the commonplace ad hoc description of the Fourier power density spectrum of X-ray timing signal. In the following study we employ the diffusion theory to directly solve for the X-ray luminosity fluctuations. The basic underlying physical assumption is that the observed variability of X-ray luminosity originates as the result of local fluctuations of the accretion rate, at all radii in the disk, that diffusively propagate outward. Energy dissipation (and X-ray emission) occurs in a narrow, shock-like region, called the transition layer, where the Keplerian ow becomes non-Keplerian in order to adjust itself to the slowly-rotating surface of a neutron star or the innermost stable orbit around a black hole. The X-ray time signal from the transition region, as seen by a remote observer, is obtained by integrating over the emission zone. The signal's power spectrum is then calculated and analyzed. Our diffusion model of the power spectrum formation operates with parameters that are physical characteristics of the accretion ow: the diffusion time scale, the Reynolds number (which is connected to the viscosity -parameter), Keplerian and magnetosonic quasi-periodic oscillation frequencies, radial size of the transition layer, and viscosity index, related to the viscosity distribution law in the system. These quantities constitute the core of temporal data used along with the spectral information to study physics of accretion. The proposed propagating fluctuation model can reproduce fundamental properties of the variability observed in X-ray light curves of accreting black hole and neutron star systems, as well as explain the power spectrum evolution during the spectral state transitions of the source.

This work focuses on the timing properties of accreting neutron stars and black holes in binary systems. It aims at development of the physical alternative to the commonplace ad hoc description of the Fourier power density spectrum of X-ray timing signal. We employ diffusion theory to directly solve for X-ray luminosity fluctuations. The basic underlying physical assumption is that the observed variability of X-ray luminosity originates as the result of local fluctuations of the mass accretion rate, at all radii in the disk, that diffusively propagate outward. Suggested diffusion model of the power spectrum formation operates with parameters that are physical characteristics of the accretion flow: the diffusion time scale, Reynolds number of the flow (which is connected to the viscosity alpha-parameter), Keplerian and magnetosonic oscillation frequencies, radial size of the transition layer, and viscosity index, related to the viscosity distribution law in the system. These quantities constitute the core of temporal data used along with the spectral information to study physics of accretion.

A collection of papers using the relative advantages of studying stellar mass and supermassive black holes. The topics discussed here include the state of the art in black hole observational and theoretical work-variability, spectroscopy, disk-jet connections, and multi-wavelength campaigns on black holes.