arXiv subjects

Vladimir I. Pariev

Publications and source records attributed to Vladimir I. Pariev.

13 recordsLinked to original sources

arXiv2020

Effect of the surface shape of a large space body on its fragmentation in a planetary atmosphere

Employing the finite element and computational fluid dynamics methods, we have determined the conditions for the fragmentation of space bodies or preservation of their integrity when they penetrate into the Earth's atmosphere. The origin of forces contributing to the fragmentation of space iron bodies during the passage through the dense layers of the planetary atmosphere has been studied. It was shown that the irregular shape of the surface can produce transverse aerodynamic forces capable of causing deformation stress in the body exceeding the tensile strength threshold of iron.

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arXiv2020

On the possibility of through passage of asteroid bodies across the Earth's atmosphere

We have studied the conditions of through passage of asteroids with diameters 200, 100 and 50 m, consisting of three types of materials -- iron, stone and water ice across the Earth's atmosphere with the minimum trajectory altitude 10--15 km. The conditions of this passage with subsequent exit into outer space with the preservation of a substantial fraction of the initial mass have been found. The results obtained support our idea explaining one of the long-standing problems of astronomy -- the Tunguska phenomenon which has not received reasonable and comprehensive interpretations to date. We argue that Tunguska event was caused by an iron asteroid body, which passed through the Earth's atmosphere and continued to the near-solar orbit.

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arXiv2006

A Magnetic Alpha-Omega Dynamo in Active Galactic Nuclei Disks: II. Magnetic Field Generation, Theories and Simulations

It is shown that a dynamo can operate in an Active Galactic Nuclei accretion disk due to the Keplerian shear and due to the helical motions of expanding and twisting plumes of plasma heated by many star passages through the disk. Each plume rotates a fraction of the toroidal flux into poloidal flux, always in the same direction, through a finite angle, and proportional to its diameter. The predicted growth rate of poloidal magnetic flux, based upon two analytic approaches and numerical simulations, leads to a rapid exponentiation of a seed field, \sim 0.1 to \sim 0.01 per Keplerian period of the inner part of the disk. The initial value of the seed field may therefore be arbitrarily small yet reach, through dynamo gain, saturation very early in the disk history. Because of tidal disruption of stars close to the black hole, the maximum growth rate occurs at a radius of about 100 gravitational radii from the central object. The generated mean magnetic field, a quadrupole field, has predominantly even parity so that the radial component does not reverse sign across the midplane. The linear growth is predicted to be the same by each of the following three theoretical analyses: the flux conversion model, the mean field approach, and numerical modeling. The common feature is the conducting fluid flow, considered in companion Paper I (Pariev and Colgate 2006) where two coherent large scale flows occur naturally: the differential winding of Keplerian motion and differential rotation of expanding plumes.

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arXiv2006

A Magnetic Alpha-Omega Dynamo in Active Galactic Nuclei Disks: I. The Hydrodynamics of Star-Disk Collisions and Keplerian Flow

A magnetic field dynamo in the inner regions of the accretion disk surrounding the supermassive black holes in AGNs may be the mechanism for the generation of magnetic fields in galaxies and in extragalactic space. We argue that the two coherent motions produced by 1) the Keplerian motion and 2) star-disk collisions, numerous in the inner region of AGN accretion disks, are both basic to the formation of a robust, coherent dynamo and consequently the generation of large scale magnetic fields. They are frequent enough to account for an integrated dynamo gain, e^{10^{9}} at 100 gravitational radii of a central black hole, many orders of magnitude greater than required to amplify any seed field no matter how small. The existence of extra-galactic, coherent, large scale magnetic fields whose energies greatly exceed all but massive black hole energies is recognized. In paper II (Pariev, Colgate, and Finn 2006) we argue that in order to produce a dynamo that can access the free energy of black hole formation and produce all the magnetic flux in a coherent fashion the existence of these two coherent motions in a conducting fluid is required. The differential winding of Keplerian motion is obvious, but the disk structure depends upon the model of "alpha", the transport coefficient of angular momentum chosen. The counter rotation of driven plumes in a rotating frame is less well known, but fortunately the magnetic effect is independent of the disk model. Both motions are discussed in this paper, paper I. The description of the two motions are preliminary to two theoretical derivations and one numerical simulation of the alpha-omega dynamo in paper II. (Abridged)

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arXiv2005

Limitations of the Hamiltonian Treatment for Collisionless Astrophysical Accretion Flows

In the Hamiltonian treatment of purely mechanical systems, the canonical and actual momentum of a particle are the same. In contrast, for a plasma of charged particles and electromagnetic fields, those two momenta are different. We show how this distinction is fundamental in identifying the limitations of a recent attempt by Binney (2003) to rule out two-temperature collisionless astrophysical accretion flows from Hamiltonian theory. This illustrates the Hamiltonian method for astrophysical plasmas, its relation to the equations of motion, and its role in practical calculations. We also discuss how the complete Hamiltonian treatment of a plasma should couple the particle motion to a fully dynamical treatment of the electromagnetic fields. Our results stand independent from the discussion of Quataert (2003) who argued that time scale calculated in Binney (2003) is not the equipartition time as claimed.

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arXiv2004

Polarization and energy content of parsec scale AGN jets

Most of energy carried by relativistic AGN jets remains undetected until hundreds of kiloparsecs where interaction with intergalactic medium produces hot spots. The jet's hidden energy is only partially dissipated at smaller scales, from parsecs to kiloparsecs. Several media may play the role of the ``prime mover'': ions, pairs or large scale magnetic fields. Analyzing VLBI polarization structures of relativistic parsec scale jets we conclude that large-scale magnetic fields can explain the salient polarization properties of parsec-scale AGN jets. This implies that large-scale magnetic fields carry a non-negligible fraction of jet luminosity. We also discuss the possibility that relativistic AGN jets may be electromagnetically (Poynting flux) dominated. In this case, dissipation of the toroidal magnetic field (and not fluid shocks) may be responsible for particle acceleration.

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arXiv2004

Polarization and structure of relativistic parsec-scale AGN jets

(Abridged) We consider the polarization properties of optically thin synchrotron radiation emitted by relativistically moving electron--positron jets carrying large-scale helical magnetic fields. In our model, the jet is cylindrical, and the emitting plasma moves parallel to the jet axis with a characteristic Lorentz factor $Γ$. We draw attention to the strong influence that the bulk relativistic motion of the emitting relativistic particles has on the observed polarization. We conclude that large-scale magnetic fields can explain the salient polarization properties of parsec-scale AGN jets. Since the typical degrees of polarization are $\leq 15%$, the emitting parts of the jets must have comparable rest-frame toroidal and poloidal fields. In this case, most relativistic jets are strongly dominated by the toroidal magnetic field component in the observer's frame, $B_ϕ/B_z \sim Γ$. We also discuss the possibility that relativistic AGN jets may be electromagnetically (Poynting flux) dominated. In this case, dissipation of the toroidal magnetic field (and not fluid shocks) may be responsible for particle acceleration.

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arXiv2003

Magnetorotational Instability in a Couette Flow of Plasma

All experiments, which have been proposed so far to model the magnetorotational instability (MRI) in the laboratory, involve a Couette flow of liquid metals in a rotating annulus. All liquid metals have small magnetic Prandtl numbers, Pm, of about 10^{-6} (the ratio of kinematic viscosity to magnetic diffusivity). With plasmas both large and small Pm are achievable by varying the temperature and the density of plasma. Compressibility and fast rotation of the plasma result in radial stratification of the equilibrium plasma density. Evolution of perturbations in radially stratified viscous and resistive plasma permeated by an axial uniform magnetic field is considered. The differential rotation of the plasma is induced by the ExB drift in applied radial electric field. Global unstable eigenmodes are calculated by our newly developed matrix code. The plasma is shown to be MRI unstable for parameters easily achievable in experimental setup.

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arXiv2002

Laminar Plasma Dynamos

A new kind of dynamo utilizing flowing laboratory plasmas has been identified. Conversion of plasma kinetic energy to magnetic energy is verified numerically by kinematic dynamo simulations for magnetic Reynolds numbers above 210. As opposed to intrinsically-turbulent liquid-sodium dynamos, the proposed plasma dynamos correspond to laminar flow topology. Modest plasma parameters, 1-20 eV temperatures, 10^{19}-10^{20} m^{-3} densities in 0.3-1.0 m scale-lengths driven by velocities on the order of the Alfven Critical Ionization Velocity (CIV), self-consistently satisfy the conditions needed for the magnetic field amplication. Growth rates for the plasma dynamos are obtained numerically with different geometry and magnetic Reynolds numbers. Magnetic-field-free coaxial plasma guns can be used to sustain the plasma flow and the dynamo.

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arXiv2000

Estimation of relativistic accretion disk parameters from iron line emission

The observed iron K-alpha fluorescence lines in Seyfert-1 galaxies provide strong evidence for an accretion disk near a supermassive black hole as a source of the emission. Here we present an analysis of the geometrical and kinematic properties of the disk based on the extreme frequency shifts of a line profile as determined by measurable flux in both the red and blue wings. The edges of the line are insensitive to the distribution of the X-ray flux over the disk, and hence provide a robust alternative to profile fitting of disk parameters. Our approach yields new, strong bounds on the inclination angle of the disk and the location of the emitting region. We apply our method to interpret observational data from MCG-6-30-15 and find that the commonly assumed inclination 30 deg for the accretion disk in MCG-6-30-15 is inconsistent with the position of the blue edge of the line at a 3 sigma level. A thick turbulent disk model or the presence of highly ionized iron may reconcile the bounds on inclination from the line edges with the full line profile fits based on simple, geometrically thin disk models. The bounds on the innermost radius of disk emission indicate that the black hole in MCG-6-30-15 is rotating faster than 30 % of theoretical maximum. When applied to data from NGC 4151, our method gives bounds on the inclination angle of the X-ray emitting inner disk of 50\pm 10 deg, consistent with the presence of an ionization cone grazing the disk as proposed by Pedlar et al. (1993). The frequency extrema analysis also provides limits to the innermost disk radius in another Seyfert-1 galaxy, NGC 3516.

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arXiv1998

Line Emission from an Accretion Disk around a Black hole: Effects of Disk Structure

The observed iron K-alpha fluorescence lines in Seyfert-1 galaxies provide strong evidence for an accretion disk near a supermassive black hole as a source of the line emission. These lines serve as powerful probes for examining the structure of inner regions of accretion disks. Previous studies of line emission have considered geometrically thin disks only, where the gas moves along geodesics in the equatorial plane of a black hole. Here we extend this work to consider effects on line profiles from finite disk thickness, radial accretion flow and turbulence. We adopt the Novikov and Thorne (1973) solution, and find that within this framework, turbulent broadening is the dominant new effect. The most prominent change in the skewed, double-horned line profiles is a substantial reduction in the maximum flux at both red and blue peaks. The effect is most pronounced when the inclination angle is large, and when the accretion rate is high. Thus, the effects discussed here may be important for future detailed modeling of high quality observational data.

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arXiv1997

Line emission from an accretion disk around black hole: effects of the disk structure

The observed iron K-alpha fluorescence lines in Seyfert galaxies provide strong evidence for an accretion disk near a supermassive black hole as a source of the line emission. Previous studies of line emission have considered only geometrically thin disks, where the gas moves along geodesics in the equatorial plane of a black hole. Here we extend this work to include effects on line profiles from finite disk thickness, radial accretion flow and turbulence. We adopt the Novikov-Thorne solution, and find that within this framework, turbulent broadening is the most significant effect. The most prominent changes in the skewed, double-horned line profiles is a substantial reduction in the maximum flux at both red and blue peaks. We show that at the present level of signal-to-noise in X-ray spectra, proper treatment of the actual structure of the accretion disk can change estimates of the inclination angle of the disk. Thus these effects will be important for future detailed modeling of high quality observational data.

astro-ph↗

arXiv1995

Hydrodynamic accretion onto rapidly rotating Kerr black hole

Bondi type hydrodynamic accretion of the surrounding matter onto Kerr black hole with an arbitrary rotational parameter is considered. The effects of viscosity, thermal conductivity and interaction with radiation field are neglected. The black hole is supposed to be at rest with respect to matter at infinity. The flow is adiabatic and has no angular momentum. The fact that usually in astrophysics substance far from the black hole has nonrelativistic temperature introduces small parameter to the problem and allows to search for the solution as a perturbation to the accretion of a cold, that is dust--like, matter. However, far from the black hole on the scales of order of the radius of the sonic surface the expansion must be performed with respect to Bondi spherically symmetrical solution for the accretion on a Newtonian gravitating centre. The equations thus obtained are solved analytically. The conditions of the regularity of the solution at the sonic surface and at infinity allow to specify unique solution, to find the shape of the sonic surface and to determine the corrections to Bondi accretion rate.

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