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1.
The results of three-dimensional MHD numerical simulations are used to investigate the characteristic properties of the magnetic-field
structures in the accretion disks of semi-detached binary systems. It is assumed that the intrinsic magnetic field of the
accretor star is dipolar. Turbulent diffusion of the magnetic field in the disk is taken into account. The SS Cyg system is
considered as an example. The results of the numerical simulations show the intense generation of a predominantly toroidal
magnetic field in the accretion disk. Magnetic zones with well defined structures for the toroidal magnetic field form in
the disk, which are separated by current sheets in which there ismagnetic reconnection and current dissipation. Possible observational
manifestations of such structures are discussed. It is shown that the interaction of a spiral precessional wave with the accretor’s
magnetosphere could lead to quasi-periodic oscillations of the accretion rate. 相似文献
2.
Typical changes of the accretion-disk structures in intermediate polars are studied as a function of the inclination of the accretor’s magnetic field. Thre-dimensional numerical modeling was performed for seven differentmagnetic-axis inclinations. The results showthat the system forms a magnetosphere region, and that column accretion occurs. The action of the magnetic field tilts the inner parts of the disk along the magnetic axis of the accretor. The magnetic-field inclination appreciably influences matter transfer in the disk and accretion processes. Generation of toroidal magnetic field, magnetic braking, and alignment of the dipole magnetic field result in oscillations of the accretion rate. A direct relationship between the field inclination and the oscillation amplitude is found, as well as an inverse relationship between the field inclination and the oscillation period. 相似文献
3.
P. B. Isakova A. G. Zhilkin D. V. Bisikalo A. N. Semena M. G. Revnivtsev 《Astronomy Reports》2017,61(7):560-572
A two-dimensional numerical model in the axisymmetric approximation that describes the flow structure in the magnetosphere of the white dwarf in the EX Hya system has been developed. Results of simulations show that the accretion in EX Hya proceeds via accretion columns, which are not closed and have curtain-like shapes. The thickness of the accretion curtains depends only weakly on the thickness of the accretion disk. This thickness developed in the simulations does not agree with observations. It is concluded that the main reason for the formation of thick accretion curtains in the model is the assumption that the magnetic field penetrates fully into the plasma of the disk. An analysis based on simple estimates shows that a diamagnetic disk that fully or partially shields the magnetic field of the star may be a more attractive explanation for the observed features of the accretion in EX Hya. 相似文献
4.
The structure and magnitude of the electric field created by a rotating accretion disk with a poloidal magnetic field is found for the case of a vacuum approximation along the axis. The accretion disk is modeled as a torus filled with plasma and a frozen-in magnetic field. The dimensions and location of the maximum electric field as well as the energy of the accelerated particles are found. The gravitational field is assumed to be weak. 相似文献
5.
An exact solution is found for the interaction of a rotating magnetic field that is frozen into a star with a thin, highly conducting accretion disk. The disk pushes the magnetic-field lines towards the star, compressing the stellar dipole magnetic field. At the corotation radius, where the Keplerian and stellar rotational frequencies are equal, a current loop appears. Electric currents flow in the magnetosphere only along two particular magnetic surfaces, which connect the corotation region and the inner edge of the disk with the stellar surface. It is shown that a closed current surface encloses the magnetosphere. The disk rotation is stopped at some distance from the stellar surface, equal to 0.55 of the corotation radius. The accretion from the disk spins up the stellar rotation. The angular momentum transferred to the star is determined. 相似文献
6.
The spins of supermassive black holes in FR I and FR II radio galaxies are estimated using two models for the generation of the relativisitic jets, based on the Blandford–Znajek and Blandford–Payne mechanisms: the hybrid model of Meier and a flux-trapping model. The magnetic field at the event horizon is estimated assuming equipartition between the energy densities of the magnetic field and the accreting material. The magnetic field near the inner edge of the accretion disk is estimated assuming equipartition between the magnetic pressure and the radiation pressure, and also assuming proportionality between the magnetic field and the spin. In the case of FR I objects, both mechanisms for the generation of the jets (the hybrid model of Meier and a flux-trapping model) are efficient. For the FR II objects, equipartition between the energy densities of the magnetic field and the accretion flow facilitates stronger retrograde rotation of the supermassive black hole. Plots of spin versus mass suggest a predominantly chaotic character for the accretion in both types of radio galaxies. 相似文献
7.
E. P. Velikhov K. R. Sychugov V. M. Chechetkin A. Yu. Lugovskii A. V. Koldoba 《Astronomy Reports》2012,56(2):84-95
We investigate the role of the magnetic field in the collapse of a gas-dust cloud into a massive gravitating object. Observations
of one such object (G31.41) indicate that the magnetic field has an hourglass shape oriented along the rotation axis of the
matter, due to the freezing-in of the magnetic-field lines in the accreting matter. It is believed that accretion in stellar
disks is associated with the transport of angular momentum from the center to the periphery, which could be initiated by large-scale
vortex structures arising in the presence of unstable rotational flows of matter. The numerical simulations have established
that the equilibrium configuration of a gas-dust disk rotating in a spherically symmetrical gravitational potential is subject
to the development of strong instability in the presence of a weak magnetic field. It is shown that the development of instability
leads to a transport of angular momentum to the disk periphery by large-scale vortex structures, together with the accretion
of matter onto the gravitating object. The magnetic-field lines near the equator take on a chaotic character, but an hourglass
configuration is observed near the rotation axis, in agreement with observations. 相似文献
8.
We have performed three-dimensional magnetohydrodynamical calculations of stream accretion in cataclysmic variable stars for
which the white dwarf primary possesses a strong, complex magnetic field. These calculations were motivated by observations
of polars: cataclysmic variables containing white dwarfs with magnetic fields sufficiently strong to prevent the formation
of an accretion disk. In this case, an accretion stream flows from the L1 point and impacts directly onto one or more spots on the surface of the white dwarf. Observations indicate that the white
dwarfs in some binaries possess complex (non-dipolar) magnetic fields. We performed simulations of ten polars, with the only
variable being the azimuthal angle of the secondary with respect to the white dwarf. These calculations are also applicable
to asynchronous polars, where the spin period of the white dwarf differs by a few percent from the orbital period. Our results
are equivalent to calculating the structure of one asynchronous polar at ten different spin-orbit beat phases. Our models
have an aligned dipolar plus quadrupolar magnetic field centered on the whitedwarf primary. We find that, with a sufficiently
strong quadrupolar component, an accretion spot arises near the magnetic equator for slightly less than half our simulations,
while a polar accretion zone is active for most of the remaining simulations. For two configurations, accretion at a dominant
polar region and in an equatorial zone occurs simultaneously. Most polar studies assume that the magnetic field is dipolar,
especially for single-pole accretors. We demonstrate that, with the orbital parameters and magnetic-field strengths typical
of polars, the accretion flow patterns can vary widely in the case of a complex magnetic field. This may make it difficult
formany polars to determine observationally whether the field is pure dipolar or is more complex, but there shoulid be indications
for some systems. In particular, a complex magnetic field should be suspected if there is an accretion zone near the white
dwarf’s equator (assumed to be in the orbital plane) or if there are two or more accretion regions that cannot be fitted by
dipolar magnetic field. Magnetic-field constraints are expected to be substantially stronger for asynchronous polars, with
clearer signs of complex field geometry due to changes in the accretion flow structure as a function of azimuthal angle. These
indications become clearer in asynchronous polars because each azimuthal angle corresponds to a different spin-orbit beat
phase. 相似文献
9.
The results of 3D modeling of the formation of the accretion disks of intermediate polars are presented. A model with misaligned rotation axes of accretor and the orbit is onsidered, in which it is assumed that the white dwarf has a dipolar magnetic field with its symmetry axis inclined to the whitedwarf rotation and orbital axes. The computations show that, in the early stages of formation of the disk, the action of magnetic field is able to create the initial (seed) inclination of the disk. This inclination is then supported mainly by the dynamical pressure of the flow from the inner Lagrangian point L1. As themass of the disk increases, the inclination disappears. Under certain conditions, the disk inclination does not arise in systems with misaligned white-dwarf rotation and orbital axes. The influence of the magnetic field and asynchronous rotation of the accretor may result in the formation of spiral waves in the disk with amplitudes sufficient to be detected observationally. 相似文献
10.
V. V. Prudskikh 《Astronomy Reports》2013,57(6):440-447
The stability of magnetohydrodynamic oscillations in a protostellar disk with a toroidal magnetic field is analyzed. It is shown that, apart from the aperiodic magnetorotational instability, two other types of periodic instabilities of non-axisymmetric perturbations can exist. The simultaneous presence of azimuthal and vertical components of the wave vector are necessary for these to exist. One instability is due to the inductive winding-up of the azimuthal magnetic field of the wave, and the other arises when the field amplitude is increased by a comoving Hall wave, transferring magnetic field into a region of enhanced field intensity. The bandwidths of the unstable wave numbers are analyzed as a function of the Hall current, the β parameter of a plasma, and the angle between the direction of wave propagation and the plane of the disk. Regions in the accretion disks typical of T Tauri stars are indentified where these instabilities could be most active. 相似文献
11.
M. Mitsumoto B. Jahanara T. Matsuda K. Oka D. V. Bisikalo E. Yu. Kilpio H. M. J. Boffin A. A. Boyarchuk O. A. Kuznetsov 《Astronomy Reports》2005,49(11):884-891
The results of three-dimensional modeling of the flow structure in the classical symbiotic system Z Andromedae are presented. Outbursts in systems of this type occur when the accretion rate exceeds the upper limit of the steady-burning range. Therefore, in order to realize the transition from a quiescent to an active state, it is necessary to find a mechanism capable of sufficiently increasing the accretion rate on the time scales typical for outburst development. Our calculations provide support for a mechanism for the transition from quiescence to outburst in classical symbiotic systems suggested earlier based on two-dimensional calculations (Bisikalo et al., 2002). Our results show that an accretion disk forms in the system for a wind velocity of 20 km s?1. The accretion rate for the solution with the disk is ～22.5–25% of the mass-loss rate of the donor, which is ～4.5?5 × 10?8M⊙ yr?1 for Z And. This value is in agreement with the steady-burning range for the white-dwarf masses usually accepted for this system. When the wind velocity increases from 20 to 30 km s?1, the accretion disk is destroyed and the disk material falls onto the accretor surface. This process is followed by an approximately twofold jump in the accretion rate. The resulting growth in the accretion rate is sufficient so as to exceed the upper limit of the steady-burning range, thus bringing the system into an active state. The time during which the accretion rate is above the steady-burning value is in very good agreement with observations. Our analysis leads us to conclude that small variations in the donor wind velocity can lead to the transition from disk accretion to wind accretion and, as a consequence, to the transition from a quiescent to an active state in classical symbiotic stars. 相似文献
12.
Results of three-dimensional numerical simulations of the gas dynamics of the envelope of the young T Tauri binary star UZ Tau E are considered. The flow structure in the circumstellar envelope of the system is analyzed. It is shown that a regime with the impulsive accretion of matter from the circumstellar disk is realized in the binary system, in which there is a periodic transfer of matter to the accretion disk of the primary component through the accretion disk of the secondary.
相似文献13.
A. E. Vol’vach L. N. Vol’vach M. G. Larionov M. F. Aller H. D. Aller M. Villata K. M. Raiteri 《Astronomy Reports》2008,52(11):867-874
Radio and optical data are used to analyze the development of the flare in the blazar 3C 454.3 observed in 2004–2007. A detailed correspondance between the optical and radio flares is established, with a time delay that depends on the observing frequency. The variation of the delay of the radio flare relative to the optical flare is opposite to the dispersion delay expected for the propagation of radiation in the interstellar medium, testifying to an intrinsic origin for the observed outburst. Small-scale flux variations on time intervals of 5–10 days in the millimeter and optical are also correlated, with a time delay of about ten months. This may provide evidence for a single source generating the radiation at all wavelengths. Rapid flux fluctuations in the radio and optical that are correlated with the indicated time delays could be associated with inhomogeneities in the accretion disk. Detailed studies of the flux variations of Active Galactic Nuclei (AGN) can be used to analyze the structure of the accretion disk. A model for the energy release in AGN that is not associated purely with accretion onto supermassive black holes is proposed. As is the case for other active members of the AGN family, estimates of the lifetime of the binary black-hole system in 3C 454.3 suggest that this object is in a stage of its evolution that is fairly close to the coalescence of its black holes. The energy that is released as the companion of the central black hole loses orbital angular momentum is sufficient to explain the observed AGN phenomena. The source of primary energy release could be heating of the gas behind shock fronts that arise due to the friction between the companion black hole and the ambient gaseous medium. The orbit of the companion could be located at the periphery of the accretion disk of the central body at its apocenter and plunge more deeply into the accretion disk at its pericenter, inducing flares at all wavelengths. Energy-release parameters such as the temperature and density of the heated gas are estimated for 3C 454.3. The model considered assumes omnidirectional radiation of the medium in the presence of a magnetic field. The radiation corresponding to the minimum flux level (base level) could represent omnidirectional radiation due to the orbit of the moving companion. The fraction of the energy that is transferred to directed jets is small, comprising 1–2% of the total energy released due to the loss of orbital angular momentum by the companion. 相似文献
14.
The formation of gaseous diffusional accretion-decretion disks is an important stage in the evolution of numerous astronomical objects. Matter is accreted onto the object in the accretion part of these disks, while the angular momentum of the accreted matter is transported from the central region to the periphery in the decretion part. Here, we consider general questions connected with the formation and evolution of diffusive accretion-decretion disks in various astrophysical objects. Such disks can be described using nonstationary diffusion models. The phenomenological parameters of these models are the coefficients in the relations for the characteristic turbulent velocity and mean free path of diffusion elements in the disk. We have developed a numerical technique to compute the disk evolution for a number of models (a massive disk, a disk with continuous accretion, a purely decretion disk). Analytical expressions estimating the basic parameters of accretion-decretion disks are presented. We discuss the relationship between the models considered and the classical α model of an accretion disk. 相似文献
15.
A. E. Vol’vach A. M. Kutkin M. G. Larionov L. N. Vol’vach A. Lahteenmaki M. Tornikoski J. Tammi J. Léon-Tavares E. Järvelä M. F. Aller H. D. Aller 《Astronomy Reports》2013,57(1):46-51
We present an analysis of data from multi-frequency monitoring of the blazar 3C 454.3 in 2010–2012, when the source experienced an unusually prolonged flare with a duration of about two years. This corresponds to the orbital period of the companion in a scenario in which two supermassive black holes are present in the nucleus of 3C 454.3. The flare’s shape, duration, and amplitude can be explained as a result of precession, if the plane of the accretion disk and the orbital plane of the binary are coincident. We detected small-scale structure of the flare, on time scales of no more than a month. These features probably correspond to inhomogeneities in the accretion disk and surrounding regions, with sizes of the order of 1015 cm. We estimated the size of the accretion disk based on the dynamical and geometrical parameters of this binary system: its diameter is comparable to the size of the orbit of the supermassive binary black hole, and its thickness does not exceed the gravitational radius of the central black hole. The presence of characteristic small-scale features during the flare makes it possible to estimate the relative time delays of variations in different spectral ranges: from gamma-ray to millimeter wavelengths. 相似文献
16.
M. Yu. Piotrovich N. A. Silant’ev Yu. N. Gnedin T. M. Natsvlishvili S. D. Buliga 《Astronomy Reports》2016,60(5):486-497
The magnetic-field structure in regions of stationary, planar accretion disks around active galactic nuclei where general-relativistic effects can be neglected (from 10 to 200 gravitational radii) is considered. It is assumed that the magnetic field in the outer edges of the disk, which forms in the magnetosphere of the central black hole during the creation of the relativisitic jets, corresponds to the field of a magnetic dipole perpendicular to the plane of the disk. In this case, the azimuthal field component Bφ in the disk arises due to the presence of the radial field Bρ and the azimuthal velocity component Uφ. The value of the magnetic field at the inner radius of the disk is taken to correspond to the solution of the induction equation in a diffusion approximation. Numerical solutions of the induction equation are given for a number of cases. 相似文献
17.
We analyze properties of the unique nova-like star AE Aquarii identified with a close binary system containing a red dwarf and a very fast rotating magnetized white dwarf. It cannot be assigned to any of the three commonly adopted sub-classes of Cataclysmic Variables: Polars, Intermediate Polars, and Accreting non-magnetized White Dwarfs. Our study has shown that the white dwarf in AE Aqr is in the ejector state and its dipole magnetic moment is ???1.5 × 1034 G cm3. It switched into this state due to intensive mass exchange between the system components during a previous epoch. A high rate of disk accretion onto the white dwarf surface resulted in temporary screening of its magnetic field and spin-up of the white dwarf to its present spin period. Transition of the white dwarf to the ejector state had occurred at a final stage of the spin-up epoch as its magnetic field emerged from the accreted plasma due to diffusion. In the frame of this scenario AE Aqr represents a missing link in the chain of Polars evolution and the white dwarf resembles a recycled pulsar. 相似文献
18.
Effects due to the interaction of the steam from the inner Lagrangian point with the accretion disk in a cataclysmic variable star are considered. The results of three-dimensional gas-dynamical numerical simulations confirm that the disk thickness in the vicinity of the interaction with the stream is minimum when the component-mass ratio is 0.6. As a consequence, some of the matter from the stream does not collide with the outer edge of the accretion disk, and continues its motion unperturbed toward the accretor. This part of the stream subsequent interacts (collides) with a thickening of the accretion disk due to the presence of a precessional wave in the disk, leading to the appearance of an additional zone of heating at the disk surface. This additional zone of enhanced luminosity (hot spot) is a direct observational manifestation of the precessional wave in the accretion disk. 相似文献
19.
ASCA, RXTE, and Chandra observations of Seyfert galaxies indicate the presence in their spectra of broad emission lines with characteristic double-peaked pro files, which could arise in the inner regions of an accretion disk. In such regions, general relativistic effects must be taken into account, and may even dominate. In connection with this, we have constructed the radiation spectrum for an individual spectral line for a model isothermal Kerr accretion disk. This demonstrates the manifestation of general relativistic effects in pure form, unclouded by effects associated with models for the structure of the disk itself. It is assumed that matter in the disk moves in circular geodesics in the equatorial plane. The spectrum retains a characteristic two-peaked profile for wide ranges of values of the radial coordinate of the radiating region, angular momentum of the black hole, and viewing angle. The inner regions of the disk make an appreciable contribution to the red wing of the spectrum. 相似文献
20.
Three-dimensional hydrodynamical modeling of the formation of the accretion disk in the SS 433 binary system is carried out with various types of cooling and numerical grids. These computations show that a thick accretion disk with a height of 0.25–0.30 (in units of the component separation) is formed around the compact object, from a flow with a large radius (0.2–0.3 in the same units) that forms in the vicinity of the inner Lagrangian point. This disk has the form of a flattened torus. The number of orbits of a particle of gas in the disk is 100–150, testifying to a minimal influence of numerical viscosity in these computations. The computations also show that the stream flowing from L1 is nearly conservative, and spirals in the disk are not formed due to the influence of the donor gravitation. 相似文献