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N. I. Shakura 《Astronomy Reports》2018,62(12):823-833
This brief review is dedicated to academician Yakov Borisovich Zeldovich, the science of his epoch and the creation of modern accretion theory. 相似文献
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A.?A.?ChashkinaEmail author P.?K.?Abolmasov A.?V.?Biryukov N.?I.?Shakura 《Astronomy Reports》2015,59(6):563-572
RXTE observations of the X-ray binary systems SXP 138, GX-304, and γ Cas in 1997–2011 have shown for the first time that these objects (X-ray binaries with Be donors) display X-ray flux variations on timescales of ～1000 days. This timescale is about 10 times longer than their orbital periods, and is comparable to the total time of the observations. The observed variations are apparently not strictly periodic and represent stochastic variability, as is characteristic of such systems in the optical. γ Cas is considered as an example. The series of optical observations of this system available in the AAVSO database covers 78 years, and is much longer than the timescale of the variability studied. Our analysis of this series has shown that γ Cas variability on a timescale of tens of years is predominantly stochastic with a power-law spectrum. 相似文献
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The instability of nonhomentropic axisymmetric flows of ideal fluid with respect to two-dimensional infinitesimal perturbations with the nonconservation of angular momentum is investigated by numerically integrating the differential equations of hydrodynamics. This problem is important in studying the dynamics of astrophysical flows as shear fluid flows around a gravitating center. A complex influence of a nonzero entropy gradient on the instability of sonic and surface gravity modes has been found. In particular, both an increase and a decrease in entropy against the effective gravity g eff causes the growth of surface gravity modes that are stable at the same parameters for a homentropic flow. At the same time, the growth rate of the sonic instability branches either monotonically increases with increasing rate of decrease in entropy against g eff or becomes zero at both negative and positive entropy gradients in the unperturbed flow. Calculations also show that growing internal gravity modes appear in the problem with free boundaries under consideration only if the flow is no longer stable with respect to axisymmetric perturbations. In addition, we show that it is improper to specify the entropy distribution in the main flow by a polytropic law with a polytropic index different from the adiabatic value, since the perturbation field does not satisfy the boundary condition at a free boundary in this case. 相似文献
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In this short paper we show that making turbulence two‐rather than three‐dimensional may increase the effective turbulent viscosity by about 40 %. Dimensionless hydrodynamical viscosity parameters up to αmax = 0.25 Mt2 may be obtained in this approach, which are in better agreement with the observational data on non‐stationary accretion than the values obtained in numerical simulations. However, the α ‐parameter values known from observations are still several times higher (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
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The instability of axisymmetric flows of ideal incompressible fluid with respect to infinitesimal perturbations with the nonconservation of angular momentum is investigated by numerically integrating the differential equations of hydrodynamics. The problem has been solved for two types of rotation profiles of an unperturbed flow: with zero and nonzero pressure gradients at the flow boundaries. Both rigid and free boundary conditions have been considered. The stability of axisymmetric flows with free boundaries is of great importance in disk accretion problems. Our calculations have revealed a crucial role of the flow pattern near the boundaries in the instability of the entire main flow. When the pressure gradient at the boundaries is zero, there is such a limiting scale of perturbations in azimuthal coordinate that longer-wavelength perturbations grow, while growing shorter-wavelength perturbations do not exit. In addition, for a fixed radial flow extent, there exists a nonzero minimum amplitude of the deviation of the angular velocity from the Keplerian one at which the instability vanishes. For a nonzero pressure gradient at the boundaries, the flow is unstable with respect to perturbations of any scale and at any small deviation of the angular velocity from the Keplerian one. 相似文献
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D. K. Klochkov N. I. Shakura K. A. Postnov R. Staubert J. Wilms N. A. Ketsaris 《Astronomy Letters》2006,32(12):804-815
We analyze and interpret the RXTE/ASM X-ray light curves for the close binary system Her X-1/HZ Her obtained from February 1996 to September 2004. Some of the features found previously in the averaged X-ray light curves are confirmed by the new RXTE/ASM data. In particular, the anomalous dips and post-eclipse recoveries in two successive orbits in the short-on state are clearly distinguishable and are stable features of the X-ray light curves. We argue that to account for these features, the tilt of the accretion disk to the orbital plane must be assumed to change with phase of the 35-day period. We present a numerical model that can reproduce the observed features of the light curves. 相似文献
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Observations of X-ray binaries indicate substantial half-thicknesses for the accretion disks in these systems (up to h/R ≈ 0.25, where h is the disk half-thickness and R its radius), while standard α accretion disks predict appreciably smaller half-thicknesses. We study the theoretical vertical structure of such disks using two independent numerical methods, and show that their maximum half-thicknesses in the subcritical regime cannot exceed h/R ≈ 0.1. We consider various reasons for the apparent increase in the disk thickness, the most probable of which is the presence of matter above the disk in the form of a hot corona that scatters hard radiation from the central source and inner parts of the disk. As a result, the observed thickness of the disk and the illumination of its outer parts effectively increase. This mechanism can also explain both the optical-to-X-ray flux ratio in these systems and the observed parameters of eclipsing X-ray binaries. 相似文献
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A model for gamma-ray bursters which naturally explains the main observational features of GRB is proposed. The GRB turned out to be linked by evolution with a well-known type of astrophysical object. 相似文献
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We have investigated the influence of X-ray irradiation on the vertical structure of the outer accretion disk in low-mass
X-ray binaries by performing a self-consistent calculation of the vertical structure and X-ray radiation transfer in the disk.
Penetrating deep into the disk, the field of scattered X-ray photons with energy E ≳ 10 keV exerts a significant influence on the vertical structure of the accretion disk at a distance R ≳ 1010 cm from the neutron star. At a distance R ∼ 1011 cm, where the total surface density in the disk reaches Σ0 ∼ 20 g cm−2, X-ray heating affects all layers of an optically thick disk. The X-ray heating effect is enhanced significantly in the presence
of an extended atmospheric layer with a temperature T
atm ≈ (2–3) × 106 K above the accretion disk. We have derived simple analytic formulas for the disk heating by scattered X-ray photons using
an approximate solution of the transfer equation by the Sobolev method. This approximation has a ≲10% accuracy in the range
of X-ray photon energies E < 20 keV. 相似文献