Generation of X-ray radiation in the inner regions of accretion disks around black holes, neutron stars, and white dwarfs

The most important temporal and spectral characteristics of X-ray radiation arising near black holes, neutron stars, and white dwarfs in the presence of matter accreting from the disk that surrounds the compact object are reviewed. It is shown how these characteristics are related to the physical parameters of these systems. A key characteristic of X-ray radiation is photon index $\Gamma$, defined as the slope of the emission spectrum in the energy range of 0.5–500 keV. If the compact object of a binary is a black hole, the X-ray radiation features saturation of the photon index (with increasing accretion rate), its value is ranging from 2 to 3. A correlation between $\Gamma$ and the quasi-periodic oscillation frequency, $\nu _{\text QPO}$, is revealed in these systems, which can be employed to independently determine the black hole mass using scaling method. The developed model of radiation transfer is now the basis of a scaling method which provides an independent estimate of mass also in the case of a supermassive black hole. The generated X-ray spectrum can be presented in a wide energy range as a combination of thermal, Comptonized, and Gaussian components that describe the emission lines. A model of radiative transfer in the vicinity of black holes and neutron stars can also explain the properties of the X-ray emission when the compact object is a white dwarf. The example of four dwarf novae, U Gem, SS Cyg, VW Hyi, and SS Aur, is used to show that the continuum of the X-ray spectrum of nonmagnetic cataclysmic variables can be described as a result of the Comptonization of soft photons on hot electrons of the accretion cloud that surrounds the white dwarf.