共查询到20条相似文献，搜索用时 718 毫秒
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Andrzej A. Zdziarski 《Monthly notices of the Royal Astronomical Society》2005,360(3):816-824
We develop a formalism to calculate energy-dependent fractional variability (rms) in accretion flows. We consider rms spectra resulting from radial dependences of the level of local variability (as expected from the propagation of disturbances in accretion flows) assuming the constant shape of the spectrum emitted at a given radius. We consider the cases when the variability of the flow is either coherent or incoherent between different radial zones. As an example of local emission, we consider blackbody, Wien and thermal Comptonization spectra. In addition to numerical results, we present a number of analytical formulae for the resulting rms. We also find an analytical formula for the disc Wien spectrum, which we find to be a very good approximation to the disc blackbody. We compare our results to the rms spectrum observed in an ultrasoft state of GRS 1915+105. 相似文献
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James M. Stone James E. Pringle 《Monthly notices of the Royal Astronomical Society》2001,322(3):461-472
We present the results of axisymmetric, time-dependent magnetohydrodynamic simulations of accretion flows around black holes. The calculations begin from a rotationally supported thick torus which contains a weak poloidal field. Accretion is produced by growth and saturation of the magnetorotational instability (MRI) provided that the wavelength of the fastest growing mode is less than the thickness of the torus. Using a computational grid that spans more than two decades in radius, we compare the time-averaged properties of the flow with previous hydrodynamical simulations. The net mass accretion rate is small compared with the mass inflow and outflow rates at large radii associated with turbulent eddies. Turbulence is driven by the MRI rather than convection. The two-dimensional structure of the time-averaged flow is significantly different compared with the hydrodynamical case. We discuss the limitations imposed on our results by the assumption of axisymmetry and the relatively small radial domain. 相似文献
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James M. Stone James E. Pringle † Mitchell C. Begelman ‡ 《Monthly notices of the Royal Astronomical Society》1999,310(4):1002-1016
Two-dimensional (axially symmetric) numerical hydrodynamical calculations of accretion flows that cannot cool through emission of radiation are presented. The calculations begin from an equilibrium configuration consisting of a thick torus with constant specific angular momentum. Accretion is induced by the addition of a small anomalous azimuthal shear stress which is characterized by a function ν . We study the flows generated as the amplitude and form of ν are varied. A spherical polar grid which spans more than two orders of magnitude in radius is used to resolve the flow over a wide range of spatial scales. We find that convection in the inner regions produces significant outward mass motions that carry away both the energy liberated by and a large fraction of the mass participating in the accretion flow. Although the instantaneous structure of the flow is complex and dominated by convective eddies, long-time averages of the dynamical variables show remarkable correspondence to certain steady-state solutions. The two-dimensional structure of the time-averaged flow is marginally stable to the Høiland criterion, indicating that convection is efficient. Near the equatorial plane, the radial profiles of the time-averaged variables are power laws with an index that depends on the radial scaling of the shear stress. A stress in which ν ∝ r 1/2 recovers the widely studied self-similar solution corresponding to an ' α -disc'. We find that, regardless of the adiabatic index of the gas, or the form or magnitude of the shear stress, the mass inflow rate is a strongly increasing function of radius, and is everywhere nearly exactly balanced by mass outflow. The net mass accretion rate through the disc is only a fraction of the rate at which mass is supplied to the inflow at large radii, and is given by the local, viscous accretion rate associated with the flow properties near the central object. 相似文献
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Santabrata Das 《Monthly notices of the Royal Astronomical Society》2007,376(4):1659-1670
We investigate the behaviour of dissipative accreting matter close to a black hole, as this provides important observational features of galactic and extragalactic black hole candidates. We find a complete set of global solutions in the presence of viscosity and synchrotron cooling. We show that advective accretion flow can have a standing shock wave and the dynamics of the shock is controlled by the dissipation parameters (both viscosity and cooling). We study the effective region of the parameter space for standing as well as oscillating shock. We find that the shock front always moves towards the black hole as the dissipation parameters are increased. However, viscosity and cooling have opposite effects in deciding the solution topologies. We obtain two critical cooling parameters that separate the nature of the accretion solution. 相似文献
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Santabrata Das Sandip K. Chakrabarti 《Monthly notices of the Royal Astronomical Society》2008,389(1):371-378
We study the dynamical structure of a cooling dominated rotating accretion flow around a spinning black hole. We show that non-linear phenomena such as shock waves can be studied in terms of only three flow parameters, namely the specific energy , the specific angular momentum (λ) and the accretion rate of the flow. We present all possible accretion solutions. We find that a significant region of the parameter space in the plane allows global accretion shock solutions. The effective area of the parameter space for which the Rankine–Hugoniot shocks are possible is maximum when the flow is dissipation-free. It decreases with the increase of cooling effects and finally disappears when the cooling is high enough. We show that shock forms further away when the black hole is rotating compared to the solution around a Schwarzschild black hole with identical flow parameters at a large distance. However, in a normalized sense, the flow parameters for which the shocks form around the rotating black holes are produced shocks closer to the black hole. The location of the shock is also dictated by the cooling efficiency in that higher the accretion rate , the closer is the shock location. We believe that some of the high-frequency quasi-periodic oscillations may be due to the flows with higher accretion rate around the rotating black holes. 相似文献
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We investigate the linear stability of a shocked accretion flow on to a black hole in the adiabatic limit. Our linear analyses and numerical calculations show that, despite the post-shock deceleration, the shock is generally unstable to non-axisymmetric perturbations. The simulation results of Molteni, Tóth & Kuznetsov can be well explained by our linear eigenmodes. The mechanism of this instability is confirmed to be based on the cycle of acoustic waves between the corotation radius and the shock. We obtain an analytical formula to calculate the oscillation period from the physical parameters of the flow. We argue that the quasi-periodic oscillation should be a common phenomenon in accretion flows with angular momentum. 相似文献
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Marek A. Abramowicz Jean-Pierre Lasota †‡ Igor V. Igumenshchev 《Monthly notices of the Royal Astronomical Society》2000,314(4):775-781
We show that recently published assertions that advection-dominated accretion flows (ADAFs) require the presence of strong winds are unfounded because they assume that low radiative efficiency in flows accreting at low rates on to black holes implies vanishing radial energy and angular momentum fluxes through the flow (which is also formulated in terms of the 'Bernoulli function' being positive). This, however, is a property only of self-similar solutions which are an inadequate representation of global accretion flows. We recall general properties of accretion flows on to black holes and show that such, necessarily transonic, flows may have either d positive or negative Bernoulli function depending on the flow viscosity. Flows with low viscosities ( α ≲0.1 in the α -viscosity model) have a negative Bernoulli function. Without exception, all 2D and 1D numerical models of low-viscosity flows constructed to date experience no significant outflows. At high viscosities the presence of outflows depends on the assumed viscosity, on the equation of state and on the outer boundary condition. The positive sign of the Bernoulli function invoked in this context is irrelevant to the presence of outflows. As an illustration, we recall 2D numerical models with moderate viscosity that have positive values of the Bernoulli function and experience no outflows. ADAFs, therefore, do not differ from this point of view from thin Keplerian discs: they may have, but they do not have to have, strong winds. 相似文献
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T. Toniazzo T. W. Hartquist R. H. Durisen 《Monthly notices of the Royal Astronomical Society》2001,322(1):149-165
We present a semi-analytical investigation of a simple one-dimensional, steady-state model for a mass-loaded, rotating, magnetized, hydrodynamical flow. Our approach is analogous to one used in early studies of magnetized winds. The model represents the infall towards a central point mass of the gas generated in a cluster of stars surrounding it, as is likely to occur in some active nuclei and starburst galaxies. We describe the properties of the different classes of infall solutions. We find that the flow becomes faster than the fast-mode speed, and hence decoupled from the centre, only for a limited range of parameter values, and when magnetic stresses are ineffective. Such flow is slowed as it approaches a centrifugal barrier, implying the existence of an accretion disc. When the flow does not become super-fast and the magnetic torque is insufficient, no steady solution extending inward to the centre exists. Finally, with a larger magnetic torque, solutions representing steady sub-Alfvénic flows are found, which can resemble spherical hydrodynamical infall. Such solutions, if applicable, would imply that rotation is not important and that any accretion disc formed would be of very limited size. 相似文献
9.
On the X-ray spectra of luminous, inhomogeneous accretion flows 总被引：1，自引：0，他引：1
A. Merloni J. Malzac A. C. Fabian R. R. Ross 《Monthly notices of the Royal Astronomical Society》2006,370(4):1699-1712
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Using two-dimensional simulations of non-radiative viscous rotating black hole accretion flows, we show that the flows with α ∼0.1–0.3 self-organize to form stationary unipolar or bipolar outflows accompanied by global meridional circulations. The required energy comes, with efficiency ∼0.001–0.01, from the matter directly accreted on to the black hole. Observational implications are discussed. 相似文献
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We study how axisymmetric magnetohydrodynamic (MHD) accretion flows depend on γ adiabatic index in the polytropic equation of state. This work is an extension of Mościbrodzka & Proga, where we investigated the γ dependence of two-dimensional Bondi-like accretion flows in the hydrodynamical (HD) limit. Our main goal is to study if simulations for various γ can give us insights into the problem of various modes of accretion observed in several types of accretion systems, such as black hole binaries (BHBs), active galactic nuclei (AGN) and gamma-ray bursts. We find that for γ≳ 4/3 , the fast-rotating flow forms a thick torus that is supported by rotation and gas pressure. As shown before for γ= 5/3 , such a torus produces a strong, persistent bipolar outflow that can significantly reduce the polar funnel accretion of a slowly rotating flow. For low γ, close to 1, the torus is thin and is supported by rotation. The thin torus produces an unsteady outflow which is too weak to propagate throughout the polar funnel inflow. Compared to their HD counterparts, the MHD simulations show that the magnetized torus can produce an outflow and does not exhibit regular oscillations. Generally, our simulations demonstrate how the torus thickness affects the outflow production. They also support the notion that the geometrical thickness of the torus correlates with the power of the torus outflow. Our results, applied to observations, suggest that the torus ability to radiatively cool and become thin can correspond to a suppression of a jet as observed in the BHBs during a transition from a hard/low to soft/high spectral state and a transition from a quiescent to hard/low state in AGN. 相似文献
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Sandip K. Chakrabarti † Santabrata Das 《Monthly notices of the Royal Astronomical Society》2001,327(3):808-812
We analytically study how the behaviour of accretion flows changes when the flow model is varied. We study the transonic properties of the conical flow, a flow of constant height and a flow in vertical equilibrium, and show that all these models are basically identical, provided that the polytropic constant is suitably changed from one model to another. We show that this behaviour is extendible even when standing shocks are produced in the flow. The parameter space where shocks are produced remains roughly identical in all these models when the same transformation among the polytropic indices is used. We present applications of these findings. 相似文献
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