22 research outputs found
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.Comment: 6 pages, 2 figures; to be published in the Proceedings of the 3d
Workshop on Non-Neutral Plasmas, July 2003, Santa Fe, US
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.Comment: 15 pages, standard LaTeX article style (submitted to Mon. Not. of the
Roy. Astron. Soc.
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 . 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 , 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, . 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.Comment: submitted to MNRAS; 45 pages, 16 figure
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.Comment: 4 pages; LaTeX; 1 figure included; uses epsfig package; to appear in
the Proceedings of the 8th Annual October Astrophysics Conference in Maryland
(Oct, 1997
First Adiabatic Invariant and the Brightness Temperature of Relativistic Jets
Assuming that the first adiabatic invariant for radiating particles in
relativistic jets is conserved, we investigate the change in brightness
temperature along the jet axis. We show that in this case the observed break in
the dependence of the brightness temperature on the distance to the central
engine can be explained
Analog of Astrophysical Magnetorotational Instability in a Couette-Taylor Flow of Polymer Fluids
We report experimental observation of an instability in a Couette-Taylor flow
of a polymer fluid in a thin gap between two coaxially rotating cylinders in a
regime where their angular velocity decreases with the radius while the
specific angular momentum increases with the radius. In the considered regime,
neither the inertial Rayleigh instability nor the purely elastic instability
are possible. We propose that the observed "elasto-rotational" instability is
an analog of the magnetorotational instability which plays a fundamental role
in astrophysical Keplerian accretion disks.Comment: 4 pages, 1 figur