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 $\Gamma$. 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, $B_\phi/B_z \sim \Gamma$. 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

Stability of a relativistic rotating electron-positron jet: nonaxisymmetric perturbations

We investigate the linear stability of a hydrodynamic relativistic flow of magnetized plasma in the force--free approximation. We considered the case of light cylindrical jet in cold and dense environment, so the jet boundary remains at rest. Continuous and discrete spectra of frequencies are investigated analytically. An infinite sequence of eigenfrequencies is found near the edge of Alfv\'en continuum. Numerical calculations showed that modes are stable and have attenuation increment $\gamma$ small. The dispersion curves $\omega =\omega (k_\parallel )$ have a minimum for $k_{{\parallel}_0}\simeq 1/R$ ($R$ is the jet radius ). This results in accumulation of perturbations inside the jet with wavelength of the order of the jet radius. The wave crests of the perturbation pattern formed in such a way move along the jet with the velocity exceeding light speed. If one has relativistic electrons emitting synchrotron radiation inside the jet, than this pattern will be visible. This provide us with the new type of superluminal source. If the jet is oriented close to the line of sight, than the observer will see knots moving backward to the core.Comment: 22 LaTeX pages, standard LaTeX article style, accepted by Mon. Not. Roy. Astron. So

Relativistic parsec-scale jets: I. Particle acceleration

We develop a theory of particle acceleration inside relativistic rotating electron-positron force-free jets with spiral magnetic fields. We considered perturbation of the stationary magnetic field structure and found that acceleration takes place in the regions where the Alfven resonant condition with the eigenmodes in the jet is fulfilled, i.e. where the local Alfven speed is equal to the phase speed of an eigenmode. The acceleration mechanism is regular acceleration by the electric field of the electromagnetic wave, which is the eigenmode of the force-free cylindrical jet: particles drift out of the region of the large wave amplitude near the Alfven resonant surface and gain energy. Acceleration in the strong electric field near the Alfven resonance and synchrotron losses combine to form a power-law energy spectrum of ultra-relativistic electrons and positrons with index between 2 and 3 depending upon the initial energy of the injected particles. The power law distribution ranges from 10 MeV to 1000 MeV.Comment: 14 pages, 4 figures; Astron. Astrophys. in pres

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